| 55531 | May.15 | Event | 1864 – American Civil War: Battle of Resaca, Georgia ends. |
| 55532 | May.15 | Event | 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. |
| 55533 | May.15 | Event | 1869 – Women's suffrage: In New York, Susan B. Anthony and Elizabeth Cady Stanton form the National Woman Suffrage Association. |
| 55534 | May.15 | Event | 1891 – Pope Leo XIII defends workers' rights and property rights in the encyclical Rerum novarum, the beginning of modern Catholic social teaching. |
| 55535 | May.15 | Event | 1904 – Russo-Japanese War: The Russian minelayer Amur lays a minefield about 15 miles off Port Arthur and sinks Japan's battleships Hatsuse, 15,000 tons, with 496 crew and Yashima. |
| 55536 | May.15 | Event | 1905 – Las Vegas, is founded when 110 acres (0.45 km2), in what later would become downtown, are auctioned off. |
| 55537 | May.15 | Event | 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. |
| 55538 | May.15 | Event | 1911 – Three hundred three Chinese and five Japanese immigrants are killed in the Torreón massacre when the forces of the Mexican Revolution led by Francisco I. Madero's brother Emilio Madero take the city of Torreón from the Federales. |
| 55539 | May.15 | Event | 1919 – The Winnipeg General Strike begins. By 11:00, almost the whole working population of Winnipeg had walked off the job. |
| 55540 | May.15 | Event | 1919 – Greek invasion of Smyrna. During the invasion, the Greek army kills or wounds 350 Turks. Those responsible are punished by the Greek Commander Aristides Stergiades. |
| 55541 | May.15 | Event | 1925 – Al-Insaniyyah, the first Arabic communist newspaper, is founded. |
| 55542 | May.15 | Event | 1928 – Walt Disney character Mickey Mouse premieres in his first cartoon, Plane Crazy. |
| 55543 | May.15 | Event | 1929 – A fire at the Cleveland Clinic in Cleveland, Ohio kills 123. |
| 55544 | May.15 | Event | 1932 – In an attempted coup d'état, the Prime Minister of Japan Inukai Tsuyoshi is assassinated. |
| 55545 | May.15 | Event | 1934 – Kārlis Ulmanis establishes an authoritarian government in Latvia. |
| 55546 | May.15 | Event | 1935 – The Moscow Metro is opened to the public. |
| 55547 | May.15 | Event | 1940 – USS Sailfish is recommissioned. It was originally the USS Squalus. |
| 55548 | May.15 | Event | 1940 – World War II: After fierce fighting, the poorly trained and equipped Dutch troops surrender to Germany, marking the beginning of five years of occupation. |
| 55549 | May.15 | Event | 1940 – McDonald's opens its first restaurant in San Bernardino, California. |
| 55550 | May.15 | Event | 1941 – First flight of the Gloster E.28/39 the first British and Allied jet aircraft. |
| 55551 | May.15 | Event | 1941 – Joe DiMaggio begins a 56-game hitting streak. |
| 55552 | May.15 | Event | 1942 – World War II: In the United States, a bill creating the Women's Army Auxiliary Corps (WAAC) is signed into law. |
| 55553 | May.15 | Event | 1943 – Joseph Stalin dissolves the Comintern (or Third International). |
| 55554 | May.15 | Event | 1945 – World War II: The Battle of Poljana, the final skirmish in Europe is fought near Prevalje, Slovenia. |
| 55555 | May.15 | Event | 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. |
| 55556 | May.15 | Event | 1951 – The Polish cultural attaché in Paris, Czesław Miłosz, asks the French government for political asylum. |
| 55557 | May.15 | Event | 1953 – Cubmaster Don Murphy organized the first pinewood derby, in Manhattan Beach, California, by Pack 280c. |
| 55558 | May.15 | Event | 1957 – At Malden Island in the Pacific Ocean, Britain tests its first hydrogen bomb in Operation Grapple. |
| 55559 | May.15 | Event | 1958 – The Soviet Union launches Sputnik 3. |
| 55560 | May.15 | Event | 1960 – The Soviet Union launches Sputnik 4. |