On this day: 21 May

Roman Emperors Diocletian and Maximian appoint Galerius as Caesar to Diocletian, beginning the period of four rulers known as the Tetrarchy.

Arab–Byzantine wars: The city of Syracuse was captured by the Aghlabids (pictured) as part of the Muslim conquest of Sicily.

Pope John VIII gives blessings to Branimir of Croatia and to the Croatian people, considered to be international recognition of the Croatian state.

Sixteen-year-old Otto III is crowned Holy Roman Emperor.

The Crusades: The siege of Shaizar ended, and the Emir of Shaizar became a vassal of the Byzantine Empire.

Coronation of Michael IX Palaiologos as Byzantine Emperor.

Coronation of John VI Kantakouzenos as Byzantine Emperor by patriarch Isidore I of Constantinople at the Church of St. Mary of Blachernae.

Dušan's Code, the constitution of the Serbian Empire, is enacted by Dušan the Mighty.

The Battle of Long Sault concludes after five days in which French colonial militia, with their Huron and Algonquin allies, are defeated by the Haudenosaunee Confederacy.

The nobility elect John Sobieski King of Poland and Grand Duke of Lithuania.

The Order of St. Alexander Nevsky is instituted in Russia by Empress Catherine I. It would later be discontinued and then reinstated by the Soviet government in 1942 as the Order of Alexander Nevsky.

The end of the Siege of Acre (1799): Napoleon Bonaparte abandons his siege of the Ottoman city of Acre after two months. This was the turning point of Bonaparte's Egyptian campaign and one of the first major defeats he suffered in his military career.

The first day of the Battle of Aspern-Essling between the Austrian army led by Archduke Charles and the French army led by Napoleon I of France sees the French attack across the Danube held.

The Congress of Colombia passed a law abolishing slavery in the country, to take effect at the beginning of the new year.

Lawrence, Kansas is captured and burned by pro-slavery forces.

A crowd of about 800 pro-slavery Americans ransacked the town of Lawrence, Kansas.

American Civil War: The Union Army succeeds in closing off the last escape route from Port Hudson, Louisiana, in preparation for the coming siege.

The Ionian Islands reunite with Greece.

Russia declares an end to the Russo-Circassian War and many Circassians are forced into exile. The day is designated the Circassian Day of Mourning.

Opening of the first rack railway in Europe, the Rigi Bahnen on Mount Rigi.

War of the Pacific: Two Chilean ships blocking the harbor of Iquique (then belonging to Peru) battle two Peruvian vessels in the Battle of Iquique.

The American Red Cross is established by Clara Barton in Dansville, New York.

The Fédération Internationale de Football Association (FIFA) is founded in Paris.

Mexican president Porfirio Díaz and the revolutionary Francisco I. Madero signed the Treaty of Ciudad Juárez to end hostilities between each other's forces, concluding the initial phase of the Mexican Revolution.

Aboard the Spirit of St. Louis, American aviator Charles Lindbergh (pictured) completed the first solo non-stop transatlantic flight, flying from Roosevelt Field near New York City to Paris–Le Bourget Airport.

Oskaloosa, Iowa, becomes the first municipality in the United States to fingerprint all of its citizens.

A Soviet station, North Pole-1, becomes the first scientific research settlement to operate on the drift ice of the Arctic Ocean.

The opening of the Ninth Street Show, otherwise known as the 9th Street Art Exhibition: A gathering of a number of notable artists, and the stepping-out of the post war New York avant-garde, collectively known as the New York School.

Michelangelo's Pietà in St. Peter's Basilica in Rome is damaged by a vandal, the mentally disturbed Hungarian geologist Laszlo Toth.

The Italian government releases the membership list of Propaganda Due, an illegal pseudo-Masonic lodge that was implicated in numerous Italian crimes and mysteries.

Transamerica Corporation agrees to sell United Artists to Metro-Goldwyn-Mayer for $380 million after the box office failure of the 1980 film Heaven's Gate.

Mengistu Haile Mariam, president of the People's Democratic Republic of Ethiopia, flees Ethiopia, effectively bringing the Ethiopian Civil War to an end.

After 30 seasons Johnny Carson hosts his penultimate episode and last featuring guests (Robin Williams and Bette Midler) of The Tonight Show.

The Democratic Republic of Yemen unsuccessfully attempts to secede from the Republic of Yemen; a war breaks out.

Indonesian president Suharto resigned after a collapse of support for his presidency amid economic and political crises, ending 32 years in power.

French Taubira law is enacted, officially recognizing the Atlantic slave trade and slavery as crimes against humanity.

The world's tallest roller coaster, Kingda Ka, opened at Six Flags Great Adventure in Jackson, New Jersey, U.S.

The Republic of Montenegro holds a referendum proposing independence from the State Union of Serbia and Montenegro; 55% of Montenegrins vote for independence.

JAXA, the Japan Aerospace Exploration Agency, launches the solar-sail spacecraft IKAROS aboard an H-IIA rocket. The vessel would make a Venus flyby late in the year.

Radio broadcaster Harold Camping predicted that the world would end on this date.

Ringling Bros. and Barnum & Bailey Circus perform their final show at Nassau Veterans Memorial Coliseum.

The Greenfield tornado, estimated to have produced winds in excess of 309 miles per hour (497 km/h), moved across Iowa.