World War II (WWII or WW2), also known as the Second World War, was a global war that lasted from 1939 to 1945. It involved the vast majority of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis. It was the most widespread war in history, with more than 100 million people serving in military units from over 30 different countries. In a state of "total war", the major participants placed their entire economic, industrial, and scientific capabilities at the service of the war effort, erasing the distinction between civilian and military resources. Marked by mass deaths of civilians, including the Holocaust and the only use of nuclear weapons in warfare, it resulted in 50 million to over 75 million fatalities. These deaths make World War II by far the deadliest conflict in human history.

The Empire of Japan aimed to dominate East Asia and was already at war with the Republic of China in 1937, but the world war is generally said to have begun on 1 September 1939 with the invasion of Poland by Germany and subsequent declarations of war on Germany by France and Britain. From late 1939 to early 1941, in a series of campaigns and treaties, Germany formed the Axis alliance with Italy, conquering or subduing much of continental Europe. Following the Molotov–Ribbentrop Pact, Germany and the Soviet Union partitioned and annexed territories between themselves of their European neighbours, including Poland. The United Kingdom and the other members of the British Commonwealth were the only major Allied forces continuing the fight against the Axis, with battles taking place in North Africa as well as the long-running Battle of the Atlantic. In June 1941, the European Axis launched an invasion of the Soviet Union, giving a start to the largest land theatre of war in history, which tied down the major part of the Axis' military forces for the rest of the war. In December 1941, Japan joined the Axis, attacked the United States and European territories in the Pacific Ocean, and quickly conquered much of the Western Pacific.