World War II, a global conflict from September 1, 1939, to September 2, 1945, pitted the Allies against the Axis powers and remains the deadliest conflict in human history. Triggered by Nazi Germany's invasion of Poland under Adolf Hitler, the war escalated dramatically when Japan attacked Pearl Harbor in December 1941, drawing the United States into the fight. The conflict saw unprecedented devastation, leading to 70 to 85 million deaths, including millions in genocides like the Holocaust, and marked the first and only use of atomic bombs on Hiroshima and Nagasaki. Following decisive Allied victories at Stalingrad and the Normandy landings, Germany surrendered in May 1945, with Japan's surrender in September 1945 officially ending the war after the atomic bombings. World War II fundamentally reshaped the geopolitical landscape, fostering the creation of the United Nations and establishing the United States and the Soviet Union as rival superpowers.