On this day: 12 April

Shapur I becomes co-emperor of the Sasanian Empire with his father Ardashir I.

Anthemius is elevated to Emperor of the Western Roman Empire.

Nikephoros I of Constantinople is consecrated as patriarch of Constantinople.

Having received the submission of the prince of Antioch, Raynald of Châtillon, Byzantine Emperor Manuel I Komnenos enters triumphantly the city of Antioch.

The Instruction of 12 April 1588 was enacted, which is widely regarded as the formal establishment of the Dutch Republic.

War of the First Coalition: Napoleon Bonaparte wins his first victory as an army commander at the Battle of Montenotte, splitting the Austrian and Piedmontese armies away from each other, and marking the beginning of the Piedmontese surrender in the war.

Alexander Ypsilantis is declared leader of Filiki Eteria, a secret organization to overthrow Ottoman rule over Greece.

American Civil War: Mobile, Alabama, falls to the Union Army.

One day after its enactment by the Congress, President William McKinley signs the Foraker Act into law, giving Puerto Rico limited self-rule.

SMS Zrínyi, one of the last pre-dreadnoughts built by the Austro-Hungarian Navy, was launched in Trieste.

World War I: Canadian forces successfully complete the taking of Vimy Ridge from the Germans.

The Bremen, a German Junkers W 33 type aircraft, takes off for the first successful transatlantic aeroplane flight from east to west.

The strongest surface wind gust in the world at the time of 231 mph, is measured on the summit of Mount Washington, New Hampshire. It has since been surpassed.

The U.S. Auto-Lite strike begins, culminating in a five-day melee between Ohio National Guard troops and 6,000 strikers and picketers.

World War II: The U.S. Ninth Army under General William H. Simpson crosses the Elbe River astride Magdeburg, and reaches Tangermünde—only 80 kilometres from Berlin.

The polio vaccine, developed by Dr. Jonas Salk, is declared safe and effective.

Aboard Vostok 1, Soviet cosmonaut Yuri Gagarin performed the first human spaceflight, completing one orbit of Earth in 108 minutes.

The Soviet nuclear-powered submarine K-33 collides with the Finnish merchant vessel M/S Finnclipper in the Danish straits.

About 1,700 gravediggers in New York City began a 12-week strike, causing a delay in the burial of roughly 1,400 bodies in the city.

Canadian athlete Terry Fox embarked on an east-to-west "Marathon of Hope" from St. John's, Newfoundland and Labrador, with a prosthetic leg to raise funds for cancer research.

The first launch of a Space Shuttle (Columbia) takes place: The STS-1 mission.

Harold Washington was elected as the first African-American mayor of Chicago.

Space Shuttle Discovery launches on STS-51D to deploy two communications satellites.

Jim Gary's "Twentieth Century Dinosaurs" exhibition opens at the Smithsonian Institution National Museum of Natural History in Washington, D.C. He is the only sculptor ever invited to present a solo exhibition there.

The Euro Disney Resort officially opens with its theme park Euro Disneyland; the resort and its park's name are subsequently changed to Disneyland Resort Paris.

Bosnian War: NATO forces began Operation Deny Flight to enforce a no-fly zone over Bosnia and Herzegovina ordered by the United Nations Security Council.

Husband-and-wife law partners Laurence Canter and Martha Siegel posted the first massive commercial spam on Usenet.

United States President Bill Clinton is cited for contempt of court for giving "intentionally false statements" in a civil lawsuit; he is later fined and disbarred.

Zimbabwe officially abandons the Zimbabwean dollar as its official currency.

The Guinea-Bissau military seized control in a coup amid a presidential election, later handing power to a transitional administration under Manuel Serifo Nhamadjo.

The Tisza Party, led by Péter Magyar wins the 2026 Hungarian parliamentary election, defeating Viktor Orbán's Fidesz party, which had ruled the country for 16 years, marking the first change in government in Hungary for over a decade.