World War II, lasting from 1939 to 1945, involved nearly every country and was marked by significant events such as the invasion of Poland by Germany, the attack on Pearl Harbor, and the use of atomic bombs on Hiroshima and Nagasaki. The war saw two main alliances: the Axis Powers led by Germany, Italy, and Japan, and the Allied Powers including the U.S., Britain, and the Soviet Union. Key battles and strategies, including blitzkrieg and island-hopping, ultimately led to the defeat of the Axis and the surrender of Germany and Japan.