1939 – World War II: France, the United Kingdom, New Zealand and Australia declare war on Germany after the invasion of Poland, forming the Allies.
. World War II, WWII or WW2, also known as the Second World War (after the recent Great War), was a global war that lasted from 1939 to 1945, though related conflicts began earlier. 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, and directly involved more than 100 million people from over 30 countries. In a 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 (during which approximately 11 million people were killed) and the strategic bombing of industrial and population centers (during which approximately one million people were killed, including the use of two nuclear weapons in combat), it resulted in an estimated 50 million to 85 million fatalities. These made World War II the deadliest conflict in human history.