World War II involved most of the world's countries—including all of the great powers—forming two opposing military alliances: the Allies and the Axis. Although fighting had broken out in Asia in 1937 and in Europe in 1939, the war is generally said to have begun on September 1, 1939, with the German invasion of Poland. The main belligerents were the United States, the United Kingdom, the Soviet Union, China, and France; all of these nations either declared war on the Axis powers or were invaded by them. The Axis powers included Germany, Italy, Japan, Hungary, Romania, Slovakia, Bulgaria, and Croatia.
The war had a profound impact on the lives and societies of people around the globe, and led to the deaths of an estimated 70 to 85 million people. The vast majority of these were civilians, killed by famine, disease, war-related accidents, or deliberate targeting. The war ended with the unconditional surrender of the Axis powers in 1945.