World War II, or the Second World War, (often abbreviated WWII or WW2) was a global military conflict which involved a majority of the world's nations, including all of the great powers, organized into two opposing military alliances: the Allies and the Axis. The war involved the mobilization of over 100 million military personnel, making it the most widespread war in history. In a state of "total war", the major participants placed their complete economic, industrial, and scientific capabilities at the service of the war effort, erasing the distinction between civilian and military resources. Over 70 million people, the majority of them civilians, were killed, making it the deadliest conflict in human history.
The start of the war is generally held to be in September 1939 with the German invasion of Poland and subsequent declarations of war on Germany by the United Kingdom, France and the British Dominions. Many belligerents entered the war before or after this date, during a period which spanned from 1937 to 1941, as a result of other events. Amongst these main events are the Marco Polo Bridge Incident, the start of Operation Barbarossa, and the attack on Pearl Harbor and British and Dutch colonies in South East Asia.
The Soviet Union and the United States emerged from the war as the world's leading superpowers. This set the stage for the Cold War, which lasted for the next 45 years. The United Nations was formed in the hope of preventing another such conflict. The self determination spawned by the war accelerated decolonisation movements in Asia and Africa, while Western Europe itself began moving toward integration.