May 20 in History

The First Council of Nicaea was a council of Christian bishops convened in the Bithynian city of Nicaea (now İznik, Turkey) by the Roman Emperor Constantine I, also known as the First Ecumenical Council. It met from May until the end of July 325. This ecumenical council was the first of many efforts to attain consensus in the church through an assembly representing all Christendom. Hosius of Corduba may have presided over its deliberations. Attended by at least 200 bishops, its main accomplishments were the settlement of the Christological issue of the divine nature of God the Son and his relationship to God the Father, the construction of the first part of the Nicene Creed, the mandating of uniform observance of the date of Easter, and the promulgation of early canon law.

Aelia Ariadne (Latin: Ariadne, Greek: Ἀριάδνη) (c. 450 – 515) was Eastern Roman empress as the wife of Zeno and Anastasius I. She is venerated as a saint in the Eastern Orthodox Church, with her feast day falling on August 22.

The Battle of Dun Nechtain or Battle of Nechtansmere (Old Irish: Cath Dúin Nechtáin; Old Welsh: Gueith Linn Garan) was fought between the Picts, led by King Bridei Mac Bili, and the Northumbrians, led by King Ecgfrith, on 20 May 685. The Northumbrian hegemony over northern Britain, won by Ecgfrith's predecessors, had begun to disintegrate. Several of Northumbria's subject nations had rebelled in recent years, leading to a number of large-scale battles against the Picts, Mercians and Irish, with varied success. After sieges of neighbouring territories carried out by the Picts, Ecgfrith led his forces against them, despite advice to the contrary, in an effort to reassert his suzerainty over the Pictish nations. A feigned retreat by the Picts drew the Northumbrians into an ambush at Dun Nechtain near the lake of Linn Garan. The battle site has long been thought to have been near the present-day village of Dunnichen in Angus. Recent research, however, has suggested a more northerly location near Dunachton, on the shores of Loch Insh in Badenoch and Strathspey. The battle ended with a decisive Pictish victory which severely weakened Northumbria's power in northern Britain. Ecgfrith was killed in battle, along with the greater part of his army. The Pictish victory marked their independence from Northumbria, who never regained their dominance in the north.

Mercia was an early medieval Anglo-Saxon kingdom, one of the seven kingdoms of the Anglo-Saxon period. It was centred on the River Trent and its tributaries, in a region now known as the Midlands of England.

Christopher Lekapenos or Lecapenus (Greek: Χριστόφορος Λακαπηνός or Λεκαπηνός, romanized: Christóphoros Lakapēnos or Lekapēnos; died August 931) was the eldest son of Emperor Romanos I Lekapenos (r. 920–944) and co-emperor of the Byzantine Empire from 921 until his death in 931. Christopher was given the position of megas hetaireiarches (commander of the palace guard) in spring 919, after Romanos assumed guardianship of the underage Emperor Constantine VII. Romanos, who made himself co-emperor in 920, raised Christopher to co-emperor on 21 May 921 to give his family precedence over Constantine VII's Macedonian line. In 928 Christopher's father-in-law, Niketas, unsuccessfully attempted to incite Christopher to usurp his father, resulting in Niketas being banished. Christopher died in August 931, succeeded by his father and two brothers, Stephen Lekapenos and Constantine Lekapenos, and Constantine VII. In December 944 his brothers overthrew and exiled their father, but they themselves were exiled in January 945 after attempting to oust Constantine VII.

A Hungarian raid in Spain took place in July 942. This was the furthest west the Hungarians raided during the period of their migration into central Europe; although, in a great raid of 924–25, the Hungarians sacked Nîmes and may have got as far as the Pyrenees.

The Second Battle of Lincoln occurred at Lincoln Castle on Saturday 20 May 1217, during the First Barons' War, between the forces of the future Louis VIII of France and those of King Henry III of England. Louis's forces were attacked by a relief force under the command of William Marshal, 1st Earl of Pembroke. Thomas, Count of Perche, commanding the French troops, was killed and Louis was expelled from his base in the southeast of England. The looting that took place afterwards is known as the "Lincoln Fair". The citizens of Lincoln were loyal to Louis so Henry's forces sacked the city.

Sancho IV of Castile (12 May 1258 – 25 April 1295) called the Brave (el Bravo), was the king of Castile, León and Galicia (now parts of Spain) from 1284 to his death. Following his brother Ferdinand's death, he gained the support of nobles who declared him king instead of Ferdinand's son Alfonso. Faced with revolts throughout his reign, before he died he made his wife regent for his son, who became Ferdinand IV.

The Battle of Alfarrobeira (Portuguese pronunciation: [alfɐʁuˈβɐjɾɐ]) took place on 20 May 1449. It was a confrontation between the forces commanded by King Afonso V of Portugal and his uncle Afonso, Duke of Braganza, against the army of the rebellious Peter, Duke of Coimbra. The place was Vialonga, near Lisbon, at the margins of the creek of Alfarrobeira. The result was the clear defeat and death of the Duke of Coimbra and the establishment of the Braganzas as the most powerful House of Portugal.

Giovanni Caboto (Italian: Giovanni Caboto [dʒoˈvanni kaˈbɔːto]; c. 1450 – c. 1499), traditionally known in English as John Cabot (English: ), was an Italian navigator and explorer. His 1497 voyage to the coast of North America under the commission of Henry VII, King of England is the earliest known European exploration of coastal North America since the Norse visits to Vinland in the eleventh century. To mark the celebration of the 500th anniversary of Cabot's expedition, both the Canadian and British governments declared Cape Bonavista, Newfoundland as representing Cabot's first landing site. However, alternative locations have also been proposed.

Vasco da Gama was a Portuguese mariner, explorer and nobleman. His discovery of the first maritime route between Europe and India—around the Cape of Good Hope, then across the Indian Ocean from Malindi in Kenya to Kozhikode—opened up European exploration of, and commerce with, India, and is considered a landmark event in world history.

Hernán Cortés, 1st Marquis of the Valley of Oaxaca was a Spanish conquistador, military commander, explorer, captain general, and writer who led an expedition that caused the fall of the Aztec Empire and brought large portions of what is now mainland Mexico under the rule of the King of Castile in the early 16th century. Cortés was part of the generation of Spanish explorers and conquistadors who began the first phase of the Spanish colonization of the Americas.

Ignatius of Loyola, venerated as Saint Ignatius of Loyola, was a Spanish Basque Catholic priest and theologian, who, with six companions, founded the religious order of the Society of Jesus (Jesuits), and became its first Superior General, in Paris in 1541.

Cartography is the study and practice of making and using maps. Combining science, aesthetics and technique, cartography builds on the premise that reality can be modeled in ways that communicate spatial information effectively.

William Shakespeare (c. 23 April 1564 – 23 April 1616) wrote sonnets on a variety of themes. When discussing or referring to Shakespeare's sonnets, it is almost always a reference to the 154 sonnets that were first published all together in a quarto in 1609. However, there are six additional sonnets that Shakespeare wrote and included in the plays Romeo and Juliet, Henry V and Love's Labour's Lost. There is also a partial sonnet found in the play Edward III.

Magdeburg ( MAG-də-burg, German: [ˈmakdəbʊʁk] , locally [ˈmaχdəbʊɐ̯ç]; Low Saxon: Meideborg [ˈmaˑɪ̯debɔɐ̯x]) is the capital and most populous city of the German state of Saxony-Anhalt. The city is on the Elbe river. The city was mentioned for the first time in a documentary record as "Magadoburg" in the Capitulary of Diedenhofen by Charlemagne in 805. Otto I, the Holy Roman Emperor and the founder of the Archbishopric of Magdeburg, was buried in the city's cathedral after his death. Magdeburg's version of German town law, known as Magdeburg rights, spread throughout Central and Eastern Europe. In the Late Middle Ages, Magdeburg was one of the largest and most prosperous German cities and a notable member of the Hanseatic League. One of the most notable people from the city was Otto von Guericke, famous for his experiments with the Magdeburg hemispheres. Magdeburg has experienced three major devastations in its history. In 1207 the first catastrophe struck the city, with a fire burning down large parts of the city, including the Ottonian cathedral. The Catholic League sacked Magdeburg in 1631, resulting in the death of 25,000 non-combatants, the largest loss of the Thirty Years' War. During World War II the Allies bombed the city in 1945 and destroyed much of the city centre. Today, around 46% of the city consists of buildings from before 1950. After World War II, the city belonged to the German Democratic Republic from 1949 to 1990. Since then, many new construction projects have been implemented and old buildings have been restored. Magdeburg celebrated its 1,200th anniversary in 2005. Magdeburg is on Autobahn 2 and Autobahn 14, connecting Eastern and Western Europe as well as northern and southern Germany. Significant industries include machines, healthcare, mechanical engineering, environmental technology, circular economy, logistics, culture, wood and information and communications technology. There are numerous cultural institutions in the city, including the Theater Magdeburg and the Museum of Cultural History. The city is also the location of two universities, the Otto von Guericke University Magdeburg and the Magdeburg-Stendal University of Applied Sciences.

The Yangzhou Massacre refers to the mass killing of people in Yangzhou, China, in May, 1645. Commanded by the Manchu prince Dodo, following the collapse of the Ming dynasty, Qing dynasty forces carried out the attack on the port city and its inhabitants, defended by remaining southern Ming forces. The massacre itself lasted a total of six days, which afterwards the burning of bodies continued. The Yangzhou Massacre served as a reminder to Ming loyalists of the threat to resisting Qing authority, and signals part of the transition from the Ming to the Qing.

Johann Sebastian Bach (31 March [O.S. 21 March] 1685 – 28 July 1750) was a German composer and musician of the late Baroque period. He is known for his prolific output across a variety of instruments and forms, including the orchestral Brandenburg Concertos; solo instrumental works such as the Cello Suites and Sonatas and Partitas for Solo Violin; keyboard works such as the Goldberg Variations and The Well-Tempered Clavier; organ works such as the Schübler Chorales and the Toccata and Fugue in D minor; and choral works such as the St. Matthew Passion and the Mass in B minor. He is known for his mastery of counterpoint, as heard in The Musical Offering and The Art of Fugue. Felix Mendelssohn precipitated the Bach Revival with a performance of the St. Matthew Passion in 1829. Ever since, Bach has been acclaimed as one of the greatest composers in the history of Western music. The Bach family had already produced several composers when Johann Sebastian was born in Eisenach, the youngest child of the city musician Johann Ambrosius Bach. After being orphaned at age 10, he lived for five years with his eldest brother, Johann Christoph, then continued his musical education in Lüneburg. In 1703 he returned to Thuringia, working as a musician for Protestant churches in Arnstadt and Mühlhausen. Around that time he also paid extended visits to the courts in Weimar, where he expanded his organ repertory, and the reformed court at Köthen, where he was mostly engaged with chamber music. By 1723 he was hired as Thomaskantor, church music director of the city of Leipzig and thus responsible for music in four Lutheran city churches and for the St. Thomas School. He decided to compose annual cycles of church cantatas, and also wrote music for Leipzig University's student ensemble, Collegium Musicum. In 1726 he began publishing his organ and other keyboard music. In Leipzig, he had difficult relations with his employer, as he had during some of his earlier positions. This situation was somewhat remedied when his sovereign, Augustus III of Poland, granted him the title of court composer of the Elector of Saxony in 1736. In the last decades of his life, Bach reworked and extended many of his earlier compositions. He died due to complications following eye surgery in 1750 at the age of 65. Four of his twenty children, Wilhelm Friedemann, Carl Philipp Emanuel, Johann Christoph Friedrich, and Johann Christian, became composers. Bach enriched established German styles through his mastery of counterpoint, harmonic and motivic organisation, and his adaptation of rhythms, forms, and textures from abroad, particularly Italy and France. His compositions include hundreds of cantatas, both sacred and secular. He composed Latin church music, Passions, oratorios, and motets. He adopted Lutheran hymns, not only in his larger vocal works but also in such works as his four-part chorales and his sacred songs. Bach wrote extensively for organ and other keyboard instruments. He composed concertos, for instance for violin and for harpsichord, and suites, as chamber music as well as for orchestra. Many of his works use contrapuntal techniques like canon and fugue. Several decades after his death, in the 18th century, Bach was still primarily known as an organist. Several biographies of Bach were published in the 19th century, and by the end of that century all of his known music had been printed. Dissemination of Bach scholarship continued through periodicals (and later websites) devoted to him, other publications such as the Bach-Werke-Verzeichnis (BWV, a numbered catalogue of his works), and new critical editions of his compositions. His music was further popularised by a multitude of arrangements, including the "Air on the G String" and "Jesu, Joy of Man's Desiring", and recordings, among them three boxed sets of performances of his complete œuvre marking the 250th anniversary of his death.

The Battle of Cartagena de Indias took place during the 1739 to 1748 War of Jenkins' Ear between Spain and Great Britain. The result of long-standing commercial tensions, the war was primarily fought in the Caribbean; the British tried to capture key Spanish ports in the region, including Porto Bello and Chagres in Panama, Havana, and Cartagena de Indias in present-day Colombia.

The Mecklenburg Declaration of Independence is a text published in 1819 with the now disputed claim that it was the first declaration of independence made in the Thirteen Colonies during the American Revolution. It was supposedly signed on May 20, 1775, in Charlotte, North Carolina, by a committee of citizens of Mecklenburg County, who declared independence from Great Britain after hearing of the battle of Concord. If true, the Mecklenburg Declaration preceded the United States Declaration of Independence by more than a year. Professional historians have maintained that the Mecklenburg Declaration of Independence is an inaccurate rendering of an authentic document known as the Mecklenburg Resolves, an argument first made by Peter Force. The Resolves, a set of radical resolutions passed on May 31, 1775, fell short of an actual declaration of independence. Although published in newspapers in 1775, the text of the Mecklenburg Resolves was lost after the American Revolution and not rediscovered until 1838. Defenders of the Mecklenburg Declaration have argued that both the Mecklenburg Declaration and the Mecklenburg Resolves are authentic. North Carolinians, convinced that the Mecklenburg Declaration was genuine, and also because of the Halifax Resolves passed by the North Carolina Provincial Congress on April 12, 1776, maintained that they were the first Americans to declare independence from Great Britain. The official seal and the flag of North Carolina display the dates of both the declaration and the Halifax vote. A holiday commemorating the Mecklenburg Declaration, "Meck Dec Day," is celebrated on May 20 in North Carolina, although it is no longer an official holiday and does not attract the attention that it once did.

In the Battle of Bautzen (20–21 May 1813), a combined Prusso-Russian army, retreating after their defeat at Lützen and massively outnumbered, was pushed back by Napoleon but escaped destruction. Some sources claim that Marshal Michel Ney failed to block their retreat. The Prussians were led by General Gebhard Leberecht von Blücher, and the Russians by General Peter Wittgenstein.

The American Civil War (April 12, 1861 – May 26, 1865; also known by other names) was a civil war in the United States between the Union ("the North") and the Confederacy ("the South"), which was formed in 1861 by states that had seceded from the Union to preserve slavery in the United States. The South saw slavery as threatened because of the election of Abraham Lincoln as US president in 1860 and the growing abolitionist movement in the North. The war ended with Union victory, the dissolution of the Confederacy and the national abolition of slavery, freeing four million African Americans. Decades of controversy over slavery came to a head when Lincoln, a Republican who opposed slavery's expansion, won the 1860 presidential election. Seven slave states in the South responded to Lincoln's victory by seceding from the United States and forming the Confederate States of America. The Confederacy seized US forts and other federal assets in the South. The war began on April 12, 1861, when the Confederacy bombarded Fort Sumter in South Carolina. A wave of enthusiasm for war swept over the North and South, as military recruitment soared. Four more Southern states seceded after the war began and, led by its president, Jefferson Davis, the Confederacy comprised eleven states, containing a third of the US population. Four years of intense combat, mostly in the South, ensued. During 1861–1862 in the western theater, the Union made permanent gains, though in the eastern theater the conflict was inconclusive. The abolition of slavery became a Union war goal on January 1, 1863, when Lincoln issued the Emancipation Proclamation, which declared all slaves in rebel states to be free, applying to more than 3.5 million of the 4 million enslaved people in the country. To the west, the Union first destroyed the Confederacy's river navy by the summer of 1862, then much of its western armies, and seized New Orleans. The successful 1863 Union siege of Vicksburg split the Confederacy in two at the Mississippi River, while Confederate general Robert E. Lee's incursion north failed at the Battle of Gettysburg. General Ulysses S. Grant's western successes led Lincoln to promote him to command of all Union armies in 1864. Inflicting an ever-tightening naval blockade of Confederate ports, the Union marshaled resources and manpower to attack the Confederacy from all directions. This led to the fall of Atlanta in 1864 to Union general William Tecumseh Sherman, followed by his March to the Sea, which culminated in his taking Savannah. The last significant battles raged around the ten-month siege of Petersburg, gateway to the Confederate capital of Richmond. The Confederates abandoned Richmond, and on April 9, 1865, Lee surrendered to Grant following the Battle of Appomattox Court House, setting in motion the end of the war. Lincoln lived to see this victory but was shot by an assassin on April 14, dying the next day. By the end of the war, much of the South's infrastructure had been destroyed. The Confederacy collapsed, the Thirteenth Amendment abolished slavery, and four million enslaved black people were freed. The war-torn nation then entered the Reconstruction era in an attempt to rebuild the country, bring the former Confederate states back into the United States, and grant civil rights to freed slaves. The war is one of the most extensively studied and written about episodes in the history of the United States. It remains the subject of cultural and historiographical debate. Of continuing interest is the myth of the Lost Cause of the Confederacy. The war was among the first to use industrial warfare. Railroads, the electrical telegraph, steamships, the ironclad warship, and mass-produced weapons were widely used. The war left an estimated 700,000 soldiers dead, along with an undetermined number of civilian deaths, making it the deadliest in American history. The technology and brutality of the Civil War foreshadowed the coming world wars.

The American Civil War (April 12, 1861 – May 26, 1865; also known by other names) was a civil war in the United States between the Union ("the North") and the Confederacy ("the South"), which was formed in 1861 by states that had seceded from the Union to preserve slavery in the United States. The South saw slavery as threatened because of the election of Abraham Lincoln as US president in 1860 and the growing abolitionist movement in the North. The war ended with Union victory, the dissolution of the Confederacy and the national abolition of slavery, freeing four million African Americans. Decades of controversy over slavery came to a head when Lincoln, a Republican who opposed slavery's expansion, won the 1860 presidential election. Seven slave states in the South responded to Lincoln's victory by seceding from the United States and forming the Confederate States of America. The Confederacy seized US forts and other federal assets in the South. The war began on April 12, 1861, when the Confederacy bombarded Fort Sumter in South Carolina. A wave of enthusiasm for war swept over the North and South, as military recruitment soared. Four more Southern states seceded after the war began and, led by its president, Jefferson Davis, the Confederacy comprised eleven states, containing a third of the US population. Four years of intense combat, mostly in the South, ensued. During 1861–1862 in the western theater, the Union made permanent gains, though in the eastern theater the conflict was inconclusive. The abolition of slavery became a Union war goal on January 1, 1863, when Lincoln issued the Emancipation Proclamation, which declared all slaves in rebel states to be free, applying to more than 3.5 million of the 4 million enslaved people in the country. To the west, the Union first destroyed the Confederacy's river navy by the summer of 1862, then much of its western armies, and seized New Orleans. The successful 1863 Union siege of Vicksburg split the Confederacy in two at the Mississippi River, while Confederate general Robert E. Lee's incursion north failed at the Battle of Gettysburg. General Ulysses S. Grant's western successes led Lincoln to promote him to command of all Union armies in 1864. Inflicting an ever-tightening naval blockade of Confederate ports, the Union marshaled resources and manpower to attack the Confederacy from all directions. This led to the fall of Atlanta in 1864 to Union general William Tecumseh Sherman, followed by his March to the Sea, which culminated in his taking Savannah. The last significant battles raged around the ten-month siege of Petersburg, gateway to the Confederate capital of Richmond. The Confederates abandoned Richmond, and on April 9, 1865, Lee surrendered to Grant following the Battle of Appomattox Court House, setting in motion the end of the war. Lincoln lived to see this victory but was shot by an assassin on April 14, dying the next day. By the end of the war, much of the South's infrastructure had been destroyed. The Confederacy collapsed, the Thirteenth Amendment abolished slavery, and four million enslaved black people were freed. The war-torn nation then entered the Reconstruction era in an attempt to rebuild the country, bring the former Confederate states back into the United States, and grant civil rights to freed slaves. The war is one of the most extensively studied and written about episodes in the history of the United States. It remains the subject of cultural and historiographical debate. Of continuing interest is the myth of the Lost Cause of the Confederacy. The war was among the first to use industrial warfare. Railroads, the electrical telegraph, steamships, the ironclad warship, and mass-produced weapons were widely used. The war left an estimated 700,000 soldiers dead, along with an undetermined number of civilian deaths, making it the deadliest in American history. The technology and brutality of the Civil War foreshadowed the coming world wars.

Abraham Lincoln (February 12, 1809 – April 15, 1865) was the 16th president of the United States, from 1861 until his assassination in 1865. He led the United States through the American Civil War, defeating the Confederacy and playing a major role in the abolition of slavery. Born in a one-room log cabin in Kentucky, Lincoln was raised on the frontier. He was self-educated and became a lawyer, Illinois state legislator, and U.S. representative. Angered by the Kansas–Nebraska Act of 1854, which opened the territories to slavery, he became a leader of the new Republican Party. He reached a national audience in the 1858 Senate campaign debates against Stephen A. Douglas. Lincoln won the 1860 presidential election, becoming the first Republican president. His victory prompted a majority of the slave states to begin seceding and form the Confederate States. A month after Lincoln assumed the presidency, Confederate forces attacked Fort Sumter, starting the Civil War. As a moderate Republican, Lincoln had to navigate conflicting political opinions from contentious factions during the war effort. He closely supervised the Union's strategy and tactics, including the selection of generals and the implementation of a naval blockade of Southern ports. He suspended the writ of habeas corpus in April 1861, an action that Chief Justice Roger Taney found in Ex parte Merryman that only Congress could do, and he averted war with Britain by defusing the Trent Affair. On January 1, 1863, he issued the Emancipation Proclamation, which declared the slaves in the states "in rebellion" to be free. On November 19, 1863, he delivered the Gettysburg Address, which became one of the most famous speeches in American history. He promoted the Thirteenth Amendment to the U.S. Constitution, which, in 1865, abolished chattel slavery. Following his re-election in 1864, he sought to heal the war-torn nation through Reconstruction. On April 14, 1865, five days after the Confederate surrender at Appomattox, Lincoln was fatally shot by John Wilkes Booth at Ford's Theatre in Washington, D.C., becoming the first U.S. president to be assassinated. Lincoln is remembered as a martyr and a national hero for his wartime leadership and for his efforts to preserve the Union and abolish slavery. He is consistently ranked in both popular and scholarly polls as among the greatest presidents in American history.

Levi Strauss ( LEE-vy STROWSS; born Löb Strauß, German: [løːp ˈʃtʁaʊs]; February 26, 1829 – September 26, 1902) was a German-born American businessman who founded the first company to manufacture blue jeans. His firm of Levi Strauss & Co. (Levi's) began in 1853 in San Francisco, California.

The Metre Convention (French: Convention du Mètre), also known as the Treaty of the Metre, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 sovereign states: Argentina, Austria-Hungary, Belgium, Brazil, Denmark, France, Germany, Italy, Peru, Portugal, Russia, Spain, Sweden-Norway, Switzerland, the Ottoman Empire, the United States, and Venezuela. The treaty created the International Bureau of Weights and Measures (BIPM), an intergovernmental organization, under the authority of the General Conference on Weights and Measures (CGPM) and the supervision of the International Committee for Weights and Measures (CIPM). These organizations coordinate international metrology and the development of internationally recognized systems of measurement. The Metre Convention established a permanent organizational structure for member governments to act in common accord on all matters relating to units of measurement. The governing organs of the BIPM are: The General Conference on Weights and Measures (Conférence générale des poids et mesures or CGPM)—the plenary organ of the BIPM which consists of the delegates of all the contracting governments, and The International Committee for Weights and Measures (Comité international des poids et mesures or CIPM)—the direction and supervision organ composed of 18 prominent metrologists from 18 different member states The headquarters or secretariat of the BIPM is at Saint-Cloud, France. It employs around 70 people and hosts BIPM's formal meetings. Initially the scope of the Metre Convention covered only units of mass and length. In 1921, at the sixth meeting of the CGPM, convention was amended to its scope to other fields in physics. In 1960, at the eleventh meeting of the CGPM, its system of units was named the International System of Units (Système international d'unités, abbreviated SI). The Metre Convention provides that only nations can be members of the BIPM. In 1999, the CGPM created in the status of associate, to allow non-member states and economic entities to participate in some activities of the BIPM through their national metrology institutes (NMIs). As of 16 October 2024, the CGPM had 64 members and 37 associates. Membership in the CGPM requires payment of substantial fees. Failure to make these payments over a span of years has led to the expulsion of some former members.

The Triple Alliance was a defensive military alliance between Germany, Austria-Hungary, and Italy. It was formed on 20 May 1882 and renewed periodically until it expired in 1915 during World War I. Germany and Austria-Hungary had been closely allied since 1879. Italy was looking for support against France shortly after it lost North African ambitions to the French. Each member promised mutual support in the event of an attack by any other great power. The treaty provided that Germany and Austria-Hungary were to assist Italy if it was attacked by France without provocation. In turn, Italy would assist Germany if attacked by France without provocation. In the event of a war between Austria-Hungary and Russia, Italy promised to remain neutral. After the Austro-Hungarian empire and Germany declared war without first being attacked by other nations, Italy did not take part in World War I on the side of the Central Powers and later joined on the side of the Allied Powers. When the treaty was renewed in February 1887, Italy gained an empty promise of German support of Italian colonial ambitions in North Africa in return for Italy's continued friendship. Austria-Hungary had to be pressured by German chancellor Otto von Bismarck into accepting the principles of consultation and mutual agreement with Italy on any territorial changes initiated in the Balkans or on the coasts and islands of the Adriatic and Aegean seas. Italy and Austria-Hungary did not overcome their basic conflict of interest in that region despite the treaty. In 1891, attempts were made to join Britain to the Triple Alliance, which, though unsuccessful, were widely believed to have succeeded in Russian diplomatic circles. Shortly after renewing the Alliance in June 1902, Italy secretly extended a similar guarantee to France. By a particular agreement, neither Austria-Hungary nor Italy would change the status quo in the Balkans without previous consultation. On 1 November 1902, five months after the Triple Alliance was renewed, Italy reached an understanding with France that each would remain neutral in the event of an attack on the other. Italy and Russia sign Racconigi Bargain in 1909 occurred shortly after which Austria-Hungary unilaterally annexed Bosnia and Herzegovina, Russia initially faced with German backing of Austria-Hungary, Russian diplomacy eventually yielded, formalized by amendments to the Treaty of Berlin in April 1909. Italy secretly acknowledged Russian interests in the Turkish Straits (Bosporus and Dardanelles). In return, Russia recognized Italian claims in Tripoli and Cyrenaica, facilitating Italy's later Tripolitanian War. The bargaining over Eastern Europe, particularly Italian recognition of Russian straits ambitions, was seen as a potential encroachment on Austrian influence in the Balkans. This tension compounded Austria-Hungary's security concerns after the loss of Russian support during the Bosnian Crisis, tightening reliance on Germany and Italy. The triple alliance members attended the Conference of the Ambassadors in 1912 after the Balkan Wars declared armistice. When the Austro-Hungarian Empire decided to start a war in August 1914 with the rival Triple Entente, Italy proclaimed its neutrality, considering Austria-Hungary the aggressor. Italy defaulted on the obligation to consult and agree to compensations before changing the status quo in the Balkans, as agreed in 1912 renewal of the Triple Alliance. Following parallel negotiation with both Triple Alliance (which aimed to keep Italy neutral) and the Triple Entente (which aimed to make Italy enter the conflict), Italy sided with the Triple Entente and declared war on Austria-Hungary. On 18 October 1883 Carol I of Romania, through his Prime Minister Ion C. Brătianu, had also secretly pledged to support the Triple Alliance, but he later remained neutral in the First World War due to viewing Austria-Hungary as the aggressor.

Krakatoa, also transcribed Krakatau, is a caldera in the Sunda Strait between the islands of Java and Sumatra in the Indonesian province of Lampung. The caldera is part of a volcanic island group comprising four islands. Two of them are known as Lang and Verlaten; another, Rakata, is the only remnant of an island, also called Krakatoa, mostly destroyed by an eruption in 1883 which created the caldera.

Cuba, officially the Republic of Cuba, is an island country in the Caribbean. It comprises the eponymous main island as well as 4,195 islands, islets, and cays. Situated at the convergence of the Caribbean Sea, Gulf of Mexico, and Atlantic Ocean, Cuba is located east of the Yucatán Peninsula, south of both Florida and the Bahamas, west of Hispaniola, and north of Jamaica and the Cayman Islands. Cuba is the largest country in the Caribbean by area and the third-largest by population, after Haiti and the Dominican Republic. Culturally, Cuba is considered to be part of Latin America. Its capital and largest city is Havana.

The 1927 Treaty of Jeddah, formally the Treaty between His Majesty and His Majesty the King of the Hejaz and of Nejd and Its Dependencies was signed between the United Kingdom and Ibn Saud.

Charles Augustus Lindbergh was an American aviator, military officer, and author. On May 20–21, 1927, he made the first nonstop flight from New York to Paris, a distance of 3,600 miles (5,800 km), flying alone for over 33 hours. His aircraft, the Spirit of St. Louis, was built to compete for the $25,000 Orteig Prize for the first flight between the two cities. Although not the first transatlantic flight, it was the first solo crossing of the Atlantic and the longest at the time by nearly 2,000 miles (3,200 km), setting a new flight distance world record. The achievement garnered Lindbergh worldwide fame and stands as one of the most consequential flights in history, signalling a new era of international air transportation.

Amelia Mary Earhart was an American aviator and aviation pioneer who became one of the most celebrated figures of early flight.

The Holocaust, known in Hebrew as the Shoah, was the genocide of European Jews during World War II. From 1941 to 1945, Nazi Germany and its collaborators systematically murdered around six million Jews across German-occupied Europe, approximately two-thirds of Europe's Jewish population. The murders were committed primarily through mass shootings across Eastern Europe and poison gas chambers in extermination camps, chiefly Auschwitz-Birkenau, Treblinka, Belzec, Sobibor, Chełmno and Majdanek death camps in occupied Poland. Concurrent Nazi persecutions killed millions of other non-Jewish civilians and prisoners of war (POWs); the term Holocaust is sometimes used to include the murder and persecution of non-Jewish groups, such as the Romani and Soviet POWs.

The Battle of Crete, codenamed Operation Mercury, was a major Axis airborne and amphibious operation during World War II to capture the island of Crete.

The Luttra Woman is a skeletonised bog body discovered in a peat bog in Falbygden near Luttra, Sweden. The remains were found on 20 May 1943 by a peat cutter. The skull was well-preserved, but some bones of the skeleton, particularly many between the skull and the pelvis, were missing. Osteological assessment identified the remains as those of a young female. The presence of raspberry seeds in her stomach contents, together with an estimated age of early to mid-twenties at death, led to her being nicknamed Hallonflickan. Radiocarbon-dated to 3928–3651 BC, she was, as of 2015, the earliest known Neolithic individual from Western Sweden. In a study, her estimated height of 145 cm was deemed short for a Stone Age woman of the region.

Chiang Kai-shek was a Chinese military commander, revolutionary, and statesman who was President of the Republic of China from 1948 to 1975 and the head of the Nationalist government from 1925 to 1948. As the de facto leader of the Republic of China (ROC), he ruled the country through World War II and oversaw the relocation of its government to Taiwan following defeat in the Chinese Civil War.

The National Security Agency (NSA) is an intelligence agency of the United States Department of Defense, under the authority of the director of national intelligence (DNI). The NSA is responsible for global monitoring, collection, and processing of information and data for intelligence and counterintelligence purposes, specializing in a discipline known as signals intelligence (SIGINT). The NSA is also tasked with the protection of U.S. communications networks and information systems. The NSA relies on a variety of measures to accomplish its mission, the majority of which are clandestine. The NSA has roughly 32,000 employees.

Operation Redwing was a United States series of 17 nuclear test detonations from May to July 1956. They were conducted at Bikini and Enewetak atolls by Joint Task Force 7 (JTF7). The entire operation followed Project 56 and preceded Project 57. The primary intention was to test new, second-generation thermonuclear weapons. Also tested were fission devices intended to be used as primaries for thermonuclear weapons, and small tactical weapons for air defense. Redwing demonstrated the first United States airdrop of a deliverable hydrogen bomb during test Cherokee. Because the yields for many tests at Operation Castle in 1954 were dramatically higher than predictions, Redwing was conducted using an "energy budget": There were limits to the total amount of energy released, and the amount of fission yield was also strictly controlled. Fission, primarily "fast" fission of the natural uranium tamper surrounding the fusion capsule, greatly increases the yield of thermonuclear devices, and constitutes the great majority of the fallout, as nuclear fusion is a relatively clean reaction.

On May 20, 1958, a Vickers Viscount airliner operating Capital Airlines Flight 300 was involved in a mid-air collision with a United States Air Force Lockheed T-33 jet trainer on a proficiency flight in the skies above Brunswick, Maryland. All 11 people on board the Viscount and one of the two crew in the T-33 were killed in the accident.

The discovery of cosmic microwave background radiation constitutes a major development in modern physical cosmology. In 1964, American physicist Arno Allan Penzias and radio-astronomer Robert Woodrow Wilson discovered the cosmic microwave background (CMB), estimating its temperature as 3.5 K, as they experimented with the Holmdel Horn Antenna. The new measurements were accepted as important evidence for a hot early Universe and as evidence against the rival steady state theory as theoretical work around 1950 showed the need for a CMB for consistency with the simplest relativistic universe models. In 1978, Penzias and Wilson were awarded the Nobel Prize for Physics for their joint measurement. There had been a prior measurement of the cosmic background radiation (CMB) by Andrew McKellar in 1941 at an effective temperature of 2.3 K using CN stellar absorption lines observed by W. S. Adams. Although no reference to the CMB is made by McKellar, it was not until much later after the Penzias and Wilson measurements, that the significance of this earlier measurement was understood.

Pakistan International Airlines Flight 705 (PK705) was a Boeing 720 airliner that crashed while descending to land at Cairo International Airport on 20 May 1965. Of the 127 passengers and crew on board, all but 6 were killed.

The Popular Movement of the Revolution was the ruling political party in Zaire. For most of its existence, it was the only legally permitted party in the country. It was founded by Joseph-Désiré Mobutu on 20 May 1967.

The Battle of Hamburger Hill was fought by United States Army and Army of the Republic of Vietnam (ARVN) forces against People's Army of Vietnam (PAVN) forces during Operation Apache Snow of the Vietnam War. Though the heavily fortified Hill 937, a ridge of the mountain Dong Ap Bia in central Vietnam near its western border with Laos, had little strategic value, US command ordered its capture by a frontal assault, only to abandon it soon thereafter. The action caused a controversy among both the US armed services and the public back home, and marked a turning point in the US involvement.

Human immunodeficiency viruses (HIVs) are two species of Lentivirus that infect humans. Over time, they cause acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive. Without treatment, the average survival time after infection with HIV is estimated to be 9 to 11 years, depending on the HIV subtype.

Radio y Televisión Martí is an international broadcaster based in Miami, Florida, financed by the federal government of the United States through the U.S. Agency for Global Media. It transmits its programs in Spanish to Cuba and its broadcasts can also be heard and viewed worldwide through its website and on shortwave radio frequencies.

Lesbian, gay, bisexual, transgender, and queer (LGBTQ) rights in the United States have developed over time, with public opinion and jurisprudence changing significantly since the late 1980s. Lesbian, gay and bisexual rights are considered advanced, but rights of transgender people have faced significant erosion since the beginning of Donald Trump's second presidency.

Timor-Leste, also known as East Timor, officially the Democratic Republic of Timor-Leste, is a country in Southeast Asia. It comprises the eastern half of the island of Timor, the coastal exclave of Oecusse in the island's northwest, and the islands of Atauro and Jaco, for a total land area of 15,007 square kilometres (5,794 sq mi). Timor-Leste shares a land border with Indonesia to the west; Australia is the country's southern neighbour, across the Timor Sea. Dili, on the north coast of Timor, is its capital and largest city.

The Indonesian Air Force is the aerial branch of the Indonesian National Armed Forces. The Indonesian Air Force is headquartered in Jakarta, Indonesia, and is headed by the Chief of Staff of the Air Force. Its order of battle is split into three Air Operations Commands. Most of its airbases are located on the island of Java. The Indonesian Air Force also has its ground force unit, called Air Force Quick Reaction Force Corps (Korpasgat). The corps is also known as the "Orange Berets" due to the distinctive color of their service headgear.

Mamata Banerjee is an Indian politician and lawyer who served as the eighth chief minister of West Bengal from 2011 to 2026. She was the first and only woman to hold that office. She founded the All India Trinamool Congress (AITC), and has previously served as a Union Cabinet Minister. She currently is the leader of the Mamata All India Trinamool Congress (MAITC), following the 2026 Trinamool Congress split.

In May 2012, two major earthquakes struck Northern Italy, causing 27 deaths and widespread damage. The events are known in Italy as the 2012 Emilia earthquakes, because they mainly affected the Emilia region.

Singapore, officially the Republic of Singapore, is an island country in Southeast Asia. Its territory comprises a main island, over 60 satellite islands and islets, and one outlying islet. The country is about one degree of latitude north of the equator, off the southern tip of the Malay Peninsula, bordering the Strait of Malacca to the west, the Singapore Strait to the south along with the Riau Islands in Indonesia, the South China Sea to the east and the Straits of Johor along with the State of Johor in Malaysia to the north.

The International System of Units, internationally known by the abbreviation SI, is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce. The International Bureau of Weights and Measures coordinates the SI.

The Russo-Ukrainian war began in February 2014 and is ongoing. Following Ukraine's Revolution of Dignity, Russia occupied Crimea and annexed it from Ukraine. It then supported Russian separatist armed groups who started a war in the eastern Donbas region against Ukraine's military. In 2018, Ukraine declared the region to be occupied by Russia. The first eight years of conflict also involved naval incidents and cyberwarfare. In February 2022, Russia launched a full-scale invasion of Ukraine and began occupying more of the country, starting the current phase of the war, the largest conflict in Europe since World War II. The war has resulted in a refugee crisis and hundreds of thousands of deaths.