World War II was a global conflict that lasted from 1939 to 1945, involving many of the world's nations. It began when Germany, led by Adolf Hitler, invaded Poland in September 1939. The war saw the rise of the Axis Powers, including Italy and Japan, against the Allied Powers, which included the United States, Soviet Union, and United Kingdom. The war resulted in significant events such as the Holocaust, where millions of Jews and others were killed, and the use of atomic bombs on Hiroshima and Nagasaki in 1945. World War II ended with the unconditional surrender of the Axis Powers, leading to major geopolitical changes and the establishment of the United Nations.