World War II was caused by political instability, economic devastation, and the rise of fascist dictators like Hitler in Germany and Mussolini in Italy. Major events included Germany invading Poland in 1939 and France in 1940, the US entering the war after the Japanese bombing of Pearl Harbor, and the dropping of atomic bombs on Hiroshima and Nagasaki in 1945. Over 40 million people in Europe died during the war, including many civilians, and the fighting devastated economies and infrastructure across the continent.