753 BC – Romulus founds Rome (traditional date).
43 BC – Battle of Mutina: Mark Antony is again defeated in battle by Aulus Hirtius, who is killed. Antony fails to capture Mutina and Decimus Brutus is murdered shortly after.
900 – The Laguna Copperplate Inscription (the earliest known written document found in what is now the Philippines): the Commander-in-Chief of the Kingdom of Tondo, as represented by the Honourable Jayadewa, Lord Minister of Pailah, pardons from all debt the Honourable Namwaran and his relations.
1092 – The Diocese of Pisa is elevated to the rank of metropolitan archdiocese by Pope Urban II
1506 – The three-day Lisbon Massacre comes to an end with the slaughter of over 1,900 suspected Jews by Portuguese Catholics.
1509 – Henry VIII ascends the throne of England on the death of his father, Henry VII.
1526 – The last ruler of the Lodi Dynasty, Ibrahim Lodi is defeated and killed by Babur in the First Battle of Panipat.
1615 – The Wignacourt Aqueduct is inaugurated in Malta.
1782 – The city of Rattanakosin, now known internationally as Bangkok, is founded on the eastern bank of the Chao Phraya River by King Buddha Yodfa Chulaloke.
1792 – Tiradentes, a revolutionary leading a movement for Brazil's independence, is hanged, drawn and quartered.
1806 – Action of 21 April 1806: A French frigate escapes British forces off the coast of South Africa.
1809 – Two Austrian army corps are driven from Landshut by a First French Empire army led by Napoleon I of France as two French corps to the north hold off the main Austrian army on the first day of the Battle of Eckmühl.
1821 – Benderli Ali Pasha arrives in Constantinople as the new Grand Vizier of the Ottoman Empire; he remains in power for only nine days before being sent into exile.
1836 – Texas Revolution: The Battle of San Jacinto: Republic of Texas forces under Sam Houston defeat troops under Mexican General Antonio López de Santa Anna.
1856 – Australian labour movement: Stonemasons and building workers on building sites around Melbourne march from the University of Melbourne to Parliament House to achieve an Eight-hour day.
1863 – Bahá'u'lláh, the founder of the Bahá'í Faith, declares his mission as "He whom God shall make manifest".
1894 – Norway formally adopts the Krag-Jørgensen bolt-action rifle as the main arm of its armed forces, a weapon that would remain in service for almost 50 years.
1898 – Spanish–American War: The United States Navy begins a blockade of Cuban ports. When the U.S. Congress issued a declaration of war on April 25, it declared that a state of war had existed from this date.
1914 – Ypiranga incident: A German arms shipment to Mexico is intercepted by the U.S. Navy near Veracruz, Veracruz.
1918 – World War I: German fighter ace Manfred von Richthofen, better known as "The Red Baron", is shot down and killed over Vaux-sur-Somme in France.
1925 – The Manifesto of the Fascist Intellectuals is published in Il Mondo, establishing the political and ideological foundations of Italian Fascism.
1934 – The "Surgeon's Photograph", the most famous photo allegedly showing the Loch Ness Monster, is published in the Daily Mail (in 1999, it is revealed to be a hoax).
1941 – Emmanouil Tsouderos becomes the 132nd Prime Minister of Greece.
1945 – World War II: Soviet forces south of Berlin at Zossen attack the German High Command headquarters.
1952 – Secretary's Day (now Administrative Professionals' Day) is first celebrated.
1960 – Brasília, Brazil's capital, is officially inaugurated. At 09:30, the Three Powers of the Republic are simultaneously transferred from the old capital, Rio de Janeiro.
1962 – The Seattle World's Fair (Century 21 Exposition) opens. It is the first World's Fair in the United States since World War II.
1963 – The Universal House of Justice of the Bahá'í Faith is elected for the first time.
1964 – A Transit-5bn satellite fails to reach orbit after launch; as it re-enters the atmosphere, 2.1 pounds (0.95 kg) of radioactive plutonium in its SNAP RTG power source is widely dispersed.
1965 – The 1964–1965 New York World's Fair opens for its second and final season.
1966 – Rastafari movement: Haile Selassie of Ethiopia visits Jamaica, an event now celebrated as Grounation Day.
1967 – Greek military junta of 1967–74: A few days before the general election in Greece, Colonel George Papadopoulos leads a coup d'état, establishing a military regime that lasts for seven years.
1970 – The Hutt River Province Principality secedes from Australia.
1975 – Vietnam War: President of South Vietnam Nguyen Van Thieu flees Saigon, as Xuan Loc, the last South Vietnamese outpost blocking a direct North Vietnamese assault on Saigon, falls.
1982 – Baseball: Rollie Fingers of the Milwaukee Brewers becomes the first pitcher to record 300 saves.
1987 – The Tamil Tigers are blamed for a car bomb that detonates in the Sri Lankan capital city of Colombo, killing 106 people.
1989 – Tiananmen Square Protests of 1989: In Beijing, around 100,000 students gather in Tiananmen Square to commemorate Chinese reform leader Hu Yaobang.
1992 – The first discoveries of extrasolar planets are announced by astronomers Alexander Wolszczan and Dale Frail. They discovered two planets orbiting the pulsar PSR 1257+12.
1993 – The Supreme Court in La Paz, Bolivia, sentences former dictator Luis Garcia Meza to 30 years in jail without parole for murder, theft, fraud and violating the constitution.
2004 – Five suicide car bombers target police stations in and around Basra, killing 74 people and wounding 160.
2010 – The controversial Kharkiv Pact (Russian Ukrainian Naval Base for Gas Treaty) is signed in Kharkiv, Ukraine, by Ukrainian President Viktor Yanukovych and Russian President Dimitry Medvedev; it will be unilaterally terminated by Russia on March 31, 2014.
2012 – Two trains are involved in a head-on collision near Sloterdijk, Amsterdam, in the Netherlands, injuring 116 people.