World War II was the largest war in human history, involving countries around the globe from 1939 to 1945. It resulted in between 45-60 million deaths. The war had several causes, including the outcome of World War I and the Treaty of Versailles, the policy of appeasement, and the rise of totalitarian regimes like Nazi Germany and fascist Italy under Hitler and Mussolini. The war began in 1939 when Germany invaded Poland.