World War II was the most devastating war in history. It lasted from 1939 to 1945 and involved most of the world's nations. Over 50 million people died during the war, including over 11 million in the Holocaust. The war was extremely costly for the US, increasing national debt from $9 billion in 1940 to $98 billion in 1945. Some of the underlying causes of the war included the harsh Treaty of Versailles after WWI, the worldwide Great Depression, the rise of totalitarian regimes like Nazi Germany and fascist Italy, and isolationist policies among Western nations that led to appeasement of aggressors like Hitler.