- August 23: Germany and the Soviet Union sign the Molotov–Ribbentrop Pact, a non-aggression pact that secretly divides Eastern Europe between the two countries.
- September 1: Germany invades Poland, starting World War II.
- September 3: Britain and France declare war on Germany.
- September 17: The Soviet Union invades Poland from the east.
- October 28: The Soviet Union and Finland sign the Moscow Peace Treaty, ending the Winter War.
1940
- April 9: Germany invades Denmark and Norway.
- May 10: Germany invades Belgium, the Netherlands, and Luxembourg.
- May 26: France surrenders to Germany.
- June 10: Italy declares war on France and Britain.
- June 22: Germany invades the Soviet Union.
- December 7: Japan attacks the American naval base at Pearl Harbor, Hawaii, bringing the United States into World War II.
1941
- January 27: The United States, Britain, and the Soviet Union sign the Declaration of the United Nations, pledging to defeat the Axis powers.
- June 22: Germany invades the Soviet Union.
- December 7: Japan attacks the American naval base at Pearl Harbor, Hawaii, bringing the United States into World War II.
1942
- February 15: The British defeat the German Afrika Korps at the Battle of El Alamein.
- April 9: The Doolittle Raid, a bombing raid on Tokyo by American aircraft, is carried out.
- June 6: D-Day, the Allied invasion of Normandy, begins.
- August 6: The United States drops an atomic bomb on Hiroshima, Japan.
- August 9: The United States drops an atomic bomb on Nagasaki, Japan.
- August 14: Japan surrenders to the Allies, ending World War II.
1943
- January 27: The siege of Leningrad ends after 900 days.
- February 2: The Battle of Stalingrad ends with a Soviet victory.
- July 25: Benito Mussolini is overthrown and arrested by the Italian king.
- September 3: Italy surrenders to the Allies.
1944
- January 22: The Anzio landings, an Allied amphibious assault on Italy, begin.
- June 6: D-Day, the Allied invasion of Normandy, begins.
- July 20: An assassination attempt is made on Adolf Hitler by a group of German officers.
- August 25: Paris is liberated by the Allies.
- December 16: The Battle of the Bulge begins, the last major German offensive of the war.
1945
- January 27: The Auschwitz concentration camp is liberated by the Soviet Army.
- February 4-11: The Yalta Conference, a meeting between the leaders of the United States, Britain, and the Soviet Union, takes place.
- April 12: President Franklin D. Roosevelt dies and is succeeded by Harry S. Truman.
- April 25: The United Nations Charter is signed in San Francisco.
- May 7: Germany surrenders to the Allies, ending World War II in Europe.
- August 6: The United States drops an atomic bomb on Hiroshima, Japan.
- August 9: The United States drops an atomic bomb on Nagasaki, Japan.
- August 14: Japan surrenders to the Allies, ending World War II.
1946
- January 1: The United Nations officially comes into existence.
- July 4: The Philippines gain independence from the United States.
- October 24: The United Nations General Assembly adopts the Universal Declaration of Human Rights.
1947
- March 12: The Truman Doctrine, a policy of containment of communism, is announced.
- June 5: The Marshall Plan, a program of economic aid to Europe, is announced.
- July 26: India gains independence from Britain.
- August 14: Pakistan gains independence from Britain.
1948
- February 25: The Communist Party of Czechoslovakia seizes power in a coup d'état.
- April 9: The Organization of American States (OAS) is established.
- May 14: The State of Israel is proclaimed.
- June 28: The Soviet Union blockades West Berlin, starting the Berlin Airlift.
1949
- January 1: The People's Republic of China is proclaimed.
- April 4: The North Atlantic Treaty Organization (NATO) is established.
- May 11: The Federal Republic of Germany (West Germany) is established.
- October 1: The German Democratic Republic (East Germany) is established.