World War II was a global military conflict that lasted from 1939 to 1945, involving most of the world's nations - including all the great powers - organized in two opposing military alliances: the Allies and the Axis. It was the most comprehensive war in history, with more than 100 million soldiers deployed. In a state of "total war", the main actors involved dedicated all their economic, industrial and scientific capacity to the service of the war effort, leaving aside the distinction between civilian and military resources.
Marked by a significant number of attacks against civilians, including the Holocaust and the only time nuclear weapons were used in combat, it was the deadliest conflict in human history, resulting in between 50 and more than 70 million deaths.
The starting point of the war is generally considered to be the invasion of Poland by Nazi Germany on September 1, 1939 and subsequent declarations of war against Germany by France and most countries of the British Empire and the Commonwealth. Some countries were already at war at this time, such as Ethiopia and the Kingdom of Italy in the Second Italo-Ethiopian War and China and Japan in the Second Sino-Japanese War.