World War 2 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 entire economic, industrial, and scientific capabilities at the service of the war effort, erasing the distinction between civilian and military resources. Over seventy million people, the majority of whom were civilians, were killed, making it the deadliest conflict in human history.
The start of the war is generally held to be September 1 1939, with the German invasion of Poland and subsequent declarations of war on Germany by most of the countries in the British Empire and Commonwealth, and by France. Many countries were already at war before this date, and many who were not initially involved joined the war later, as a result of events such as the Marco Polo Bridge Incident (fought between Nationalist China and Japan), the German invasion of the Soviet Union, and the attacks on Pearl Harbor and British and Dutch colonies in Southeast Asia.
In 1945 the war ended in a victory for the Allies. The Soviet Union and the United States subsequently emerged as the world's superpowers, setting 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 acceptance of the principle of self-determination accelerated decolonization movements in Asia and Africa, while Western Europe itself began moving toward integration.