May 15 is the 135th day of the year (136th in leap years) in the Gregorian calendar; 230 days remain until the end of the year.
756 – Abd al-Rahman I, the founder of the Arab dynasty that ruled the greater part of Iberia for nearly three centuries, becomes emir of Cordova, Spain
1536 – Anne Boleyn, Queen of England, stands trial in London on charges of treason, adultery and incest; she is condemned to death by a specially-selected jury.
1602 – Cape Cod is sighted by English navigator Bartholomew Gosnold.
1817 – Opening of the first private mental health hospital in the United States, the Asylum for the Relief of Persons Deprived of the Use of Their Reason (now Friends Hospital, Philadelphia, Pennsylvania).
1851 – The first Australian gold rush is proclaimed, although the discovery had been made three months earlier.
1864 – American Civil War: Battle of New Market, Virginia: Students from the Virginia Military Institute fight alongside the Confederate army to force Union General Franz Sigel out of the Shenandoah Valley.
1905 – The city of Las Vegas is founded in Nevada, United States
1911 – In Standard Oil Co. of New Jersey v. United States, the United States Supreme Court declares Standard Oil to be an 'unreasonable' monopoly under the Sherman Antitrust Act and orders the company to be broken up.
1911 – More than 300 Chinese immigrants are killed in the Torreón massacre when the forces of the Mexican Revolution led by Emilio Madero take the city of Torreón from the Federales.
1919 – Greek occupation of Smyrna. During the occupation, the Greek army kills or wounds 350 Turks; those responsible are punished by Greek commander Aristides Stergiades.
1929 – A fire at the Cleveland Clinic in Cleveland, Ohio kills 123.
1933 – All military aviation organizations within or under the control of the RLM of Germany were officially merged in a covert manner to form its Wehrmacht military's air arm, the Luftwaffe.
1940 – World War II: The Battle of the Netherlands: After fierce fighting, the poorly trained and equipped Dutch troops surrender to Germany, marking the beginning of five years of occupation.
1940 – Richard and Maurice McDonald open the first McDonald's restaurant.
1942 – World War II: In the United States, a bill creating the Women's Army Auxiliary Corps (WAAC) is signed into law.
1943 – Joseph Stalin dissolves the Comintern (or Third International).
1945 – World War II: The Battle of Poljana, the final skirmish in Europe is fought near Prevalje, Slovenia.
1948 – Following the expiration of The British Mandate for Palestine, the Kingdom of Egypt, Transjordan, Lebanon, Syria, Iraq and Saudi Arabia invade Israel thus starting the 1948 Arab– Israeli War.
1957 – At Malden Island in the Pacific Ocean, Britain tests its first hydrogen bomb in Operation Grapple.
1963 – Project Mercury: The launch of the final Mercury mission, Mercury-Atlas 9 with astronaut Gordon Cooper on board. He becomes the first American to spend more than a day in space, and the last American to go into space alone.
1970 – President Richard Nixon appoints Anna Mae Hays and Elizabeth P. Hoisington the first female United States Army generals.
1974 – Ma'alot massacre: Members of the Democratic Front for the Liberation of Palestine attack and take hostages at an Israeli school; a total of 31 people are killed, including 22 schoolchildren.
1988 – Soviet–Afghan War: After more than eight years of fighting, the Soviet Army begins to withdraw 115,000 troops from Afghanistan.
1991 – Édith Cresson becomes France's first female Prime Minister.
1997 – The United States government acknowledges the existence of the 'Secret War' in Laos and dedicates the Laos Memorial in honor of Hmong and other 'Secret War' veterans.
2001 – A CSX EMD SD40-2 rolls out of a train yard in Walbridge, Ohio, with 47 freight cars, including some tank cars with flammable chemical, after its engineer fails to reboard it after setting a yard switch. It travels south driverless for 66 miles (106 km) until it was brought to a halt near Kenton, Ohio. The incident became the inspiration for the 2010 film Unstoppable.
2008 – California becomes the second U.S. state after Massachusetts in 2004 to legalize samesex marriage after the state's own Supreme Court rules a previous ban unconstitutional.
2010 – Jessica Watson becomes the youngest person to sail, non-stop and unassisted around the world solo.
2013 – An upsurge in violence in Iraq leaves more than 389 people dead over three days.