World War II was the most destructive war in human history.
It began in 1939 when Germany invaded Poland, but eventually grew to include most of the countries of the world.
In Europe, Germany fought England and the Union of Soviet Socialist Republics (USSR); in the Pacific, the United States fought Japan. The most horrible atrocity of the war was the systematic, racist killing of 6 million Jews by the German Nazis in Europe.
Some 45 million people lost their lives in the war, which ended in 1945, leaving a new world order dominated by the United States and the USSR.