World War II lasted from 1939 to 1945, with significant human costs resulting in 50 million deaths and immense financial burdens, including a U.S. debt increase from $9 billion in 1940 to $98 billion by 1945. The conflict saw major powers divided into the Allies and Axis, with notable leaders such as Adolf Hitler and Franklin D. Roosevelt playing crucial roles. The war was precipitated by underlying causes, such as the Treaty of Versailles and isolationism among major countries, which ultimately led to policies of appeasement.