The Second World War is so far the only truly global war that has ever taken place. It involved the vast majority of the world's nations, with 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 from over thirty different countries serving in military units. In this state of 'total war', the major participants threw their entire economic, industrial and scientific capabilities behind 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 an estimated 80 million fatalities. All of this made the Second World War the deadliest conflict in human history. This introduction to the Second World War follows the major events that led up to the war and occurred during it, year by year.