325 – The First Council of Nicea is formally opened, starting the first Ecumenical Council of the Christian Church.
491 – Empress Ariadne marries Anastasius I. The widowed Augusta is able to choose her successor for the Byzantine throne, after Zeno (late emperor) dies of dysentery.
526 – An earthquake kills about 250,000 people in what is now Syria and Antiochia.
685 – The Battle of Dun Nechtain is fought between a Pictish army under King Bridei III and the invading Northumbrians under King Ecgfrith, who are decisively defeated.
794 – King Æthelberht II of East Anglia visits the royal Mercian court at Sutton Walls, with a view to marrying princess Ælfthryth. He is taken captive and beheaded.
1217 – The Second Battle of Lincoln is fought near Lincoln, England, resulting in the defeat of Prince Louis of France by William Marshal, 1st Earl of Pembroke.
1293 – King Sancho IV of Castile creates the Study of General Schools of Alcalá.
1449 – The Battle of Alfarrobeira is fought, establishing the House of Braganza as a principal royal family of Portugal.
1497 – John Cabot sets sail from Bristol, England, on his ship Matthew looking for a route to the west (other documents give a May 2 date).
1498 – Portuguese explorer Vasco da Gama discovers the sea route to India when he arrives at Kozhikode (previously known as Calicut), India.
1520 – The massacre at the festival of Tóxcatl takes place during the Fall of Tenochtitlan, resulting in turning the Aztecs against the Spanish.
1521 – Ignatius Loyola is seriously wounded in the Battle of Pampeluna.
1570 – Cartographer Abraham Ortelius issues Theatrum Orbis Terrarum, the first modern atlas.
1609 – Shakespeare’s sonnets are first published in London, perhaps illicitly, by the publisher Thomas Thorpe.
1631 – The city of Magdeburg in Germany is seized by forces of the Holy Roman Empire and most of its inhabitants massacred, in one of the bloodiest incidents of the Thirty Years’ War.
1645 – Yangzhou massacre: the 10-day massacre of 800,000 residents of the city of Yangzhou, part of the Transition from Ming to Qing.
1775 – Mecklenburg Declaration of Independence is signed in Charlotte, North Carolina
1802 – By the Law of 20 May 1802, Napoleon Bonaparte reinstates slavery in the French colonies, revoking its abolition in the French Revolution.
1813 – Napoleon Bonaparte leads his French troops into the Battle of Bautzen in Saxony, Germany, against the combined armies of Russia and Prussia. The battle ends the next day with a French victory.
1840 – York Minster is badly damaged by fire.
1861 – American Civil War: The state of Kentucky proclaims its neutrality, which will last until September 3 when Confederate forces enter the state. Meanwhile, the State of North Carolina secedes from the Union.
1862 – U.S. President Abraham Lincoln signs the Homestead Act into law.
1864 – American Civil War: Battle of Ware Bottom Church: In the Virginia Bermuda Hundred Campaign, 10,000 troops fight in this Confederate victory.
1873 – Levi Strauss and Jacob Davis receive a U.S. patent for blue jeans with copper rivets.
1875 – Signing of the Metre Convention by 17 nations leading to the establishment of the International System of Units.
1882 – The Triple Alliance between the German Empire, Austria-Hungary and the Kingdom of Italy is formed.
1883 – Krakatoa begins to erupt; the volcano explodes three months later, killing more than 36,000 people.
1884 – Dinuzulu kaCetshwayo becomes the king of the Zulu Nation.
1891 – History of Cinema: The first public display of Thomas Edison’s prototype kinetoscope.
1896 – The six ton chandelier of the Palais Garnier falls on the crowd below resulting in the death of one and the injury of many others.
1899 – The first traffic ticket in the US: New York City taxi driver Jacob German was arrested for speeding while driving 12 miles per hour on Lexington Street.
1902 – Cuba gains independence from the United States. Tomás Estrada Palma becomes the country’s first President.
1908 – Budi Utomo organization is founded in Dutch East Indies, beginning the Indonesian National Awakening.
1916 – The Saturday Evening Post publishes its first cover with a Norman Rockwell painting (Boy with Baby Carriage).
1920 – Montreal radio station XWA broadcasts the first regularly scheduled radio programming in North America.
1927 – Treaty of Jeddah: The United Kingdom recognizes the sovereignty of King Ibn Saud in the Kingdoms of Hejaz and Nejd, which later merge to become the Kingdom of Saudi Arabia.
1927 – At 07:52 Charles Lindbergh takes off from Roosevelt Field in Long Island, New York, on the world’s first solo non-stop flight across the Atlantic Ocean. He touched down at Le Bourget Field in Paris at 22:22 the next day.
1932 – Amelia Earhart takes off from Newfoundland to begin the world’s first solo nonstop flight across the Atlantic Ocean by a female pilot, landing in Ireland the next day.
1940 – The Holocaust: The first prisoners arrive at a new concentration camp at Auschwitz.
1941 – World War II: Battle of Crete: German paratroops invade Crete.
1948 – Chiang Kai-shek is elected as the first President of the Republic of China.
1949 – In the United States, the Armed Forces Security Agency, the predecessor to the National Security Agency, is established.
1956 – In Operation Redwing, the first United States airborne hydrogen bomb is dropped over Bikini Atoll in the Pacific Ocean.
1964 – Discovery of the cosmic microwave background radiation by Robert Woodrow Wilson and Arno Penzias.
1965 – PIA Flight 705, a Pakistan International Airlines Boeing 720-040B, crashes while descending to land at Cairo International Airport, killing 121 of the 127 passengers and crew.
1967 – The Popular Movement of the Revolution political party is established in the Democratic Republic of the Congo.
1969 – The Battle of Hamburger Hill in Vietnam ends.
1980 – In a referendum in Quebec, the population rejects by a 60% vote the proposal from its government to move towards independence from Canada.
1983 – First publications of the discovery of the HIV virus that causes AIDS in the journal Science by Luc Montagnier.
1983 – Church Street bombing: A car bomb planted by Umkhonto we Sizwe explodes on Church Street in South Africa’s capital, Pretoria, killing 19 people and injuring 217 others.
1985 – Radio Martí, part of the Voice of America service, begins broadcasting to Cuba.
1989 – The Chinese authorities declare martial law in the face of pro-democracy demonstrations, setting the scene for the Tiananmen Square massacre.
1990 – The first post-Communist presidential and parliamentary elections are held in Romania.
1996 – Civil rights: The Supreme Court of the United States rules in Romer v. Evans against a law that would have prevented any city, town or county in the state of Colorado from taking any legislative, executive, or judicial action to protect the rights of gays and lesbians.
2002 – The independence of East Timor is recognized by Portugal, formally ending 23 years of Indonesian rule and three years of provisional UN administration (Portugal itself is the former colonizer of East Timor until 1976).
2006 – A series of massive strikes begin involving nearly 1.8 million garment workers in Bangladesh.
2010 – A social media event is celebrated in response to the Muslim extremists’ reaction to images of Mohammed.
2012 – At least 27 people are killed and 50 others injured when a 6.0-magnitude earthquake strikes northern Italy.
2013 – An EF5 tornado strikes the Oklahoma City suburb of Moore, killing 24 people and injuring 377 others.
2014 – More than 118 people are killed in two bombings in Jos, Nigeria.