- September 1: Germany invades Poland, starting World War II.
- September 3: Britain and France declare war on Germany.
- September 17: Soviet Union invades Poland from the east.
- September 28: Warsaw surrenders to the Germans.
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, Italy, and Japan sign the Tripartite Pact, forming the Axis Powers.
- July 10: The Battle of Britain begins.
- September 15: The Battle of Britain ends with a British victory.
1941
- December 7: Japan attacks Pearl Harbor, bringing the United States into World War II.
- December 11: Germany and Italy declare war on the United States.
- December 19: The Philippines surrenders to the Japanese.
- January 27: The Siege of Leningrad begins.
1942
- February 15: The Fall of Singapore.
- April 9: The Bataan Death March begins.
- May 27: The Battle of Midway begins.
- June 6: D-Day: Allied forces land in Normandy, France.
- July 25: The Battle of Stalingrad begins.
- October 23: The Battle of El Alamein begins.
- November 8: Allied forces land in North Africa.
1943
- January 27: The Siege of Leningrad ends.
- February 2: The Battle of Stalingrad ends with a Soviet victory.
- May 13: The Battle of El Alamein ends with an Allied victory.
- July 25: Allied forces land in Sicily.
- September 3: Italy surrenders to the Allies.
- October 20: The Battle of Leyte Gulf begins.
- November 24: The Cairo Conference takes place.
1944
- January 22: Allied forces land at Anzio, Italy.
- June 6: D-Day: Allied forces land in Normandy, France.
- July 20: The July 20 Plot to assassinate Hitler fails.
- August 1: Warsaw Uprising begins.
- August 25: Paris is liberated by Allied forces.
- September 17: The Battle of Arnhem begins.
- September 23: The Battle of Arnhem ends with a German victory.
- December 16: The Battle of the Bulge begins.
1945
- January 27: The Auschwitz concentration camp is liberated by Soviet forces.
- February 4-11: The Yalta Conference takes place.
- February 13: The Dresden bombing begins.
- March 7: The Battle of Iwo Jima begins.
- March 9: The Battle of Iwo Jima ends with an American victory.
- March 10: The Tokyo firebombing begins.
- April 12: President Franklin D. Roosevelt dies; Harry S. Truman becomes president.
- April 16: The Battle of Berlin begins.
- April 30: Hitler commits suicide.
- May 7: Germany surrenders to the Allies.
- 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.
- September 2: World War II ends.