World War II was a global military conflict that lasted from 1939 to 1945, involving most of the world's nations. It was the deadliest and most widespread war in history, resulting in an estimated 70-85 million casualties, including civilians. The war was fought between two major alliances: the Allies, led by the United States, United Kingdom, and Soviet Union; and the Axis powers, led by Nazi Germany, Italy, and Japan. Key events of World War II include the invasion of Poland, the Battle of Stalingrad, the bombing of Hiroshima and Nagasaki, and the D-Day invasion of Normandy. The war ended with the unconditional surrender of the Axis powers in 1945.