World War II was a global war that lasted from 1939 to 1945. It involved over 30 countries and 100 million people, making it the largest and deadliest war in history. The war was fought between two military alliances - the Allies (including countries like the US, UK, and Soviet Union) and the Axis powers (led by Germany, Italy and Japan). The war resulted in around 50-85 million deaths worldwide and saw the rise of totalitarian regimes and the Holocaust.