World War II, or the Second World War[1] (often abbreviated WWII or WW2), was a global military conflict between 1939 and 1945, which involved most of the world's nations, including all great powers, organised into two opposing military alliances: the Allies and the Axis. It was the most widespread war in history, with more than 100 million military personnel mobilised. 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 civilians, were killed, making it the deadliest conflict in human history.[2]

The start of the war is generally held to be September 1, 1939, with the invasion of Poland by Nazi Germany 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, such as Ethiopia and Italy in the Second Italo-Abyssinian War and China and Japan in the Second Sino-Japanese War.[3] Many who were not initially involved joined the war later, as a result of events such as the German invasion of the Soviet Union, the Japanese attacks on Pearl Harbor and British colonies, and subsequent declarations of war on Japan by the United States, the Netherlands,[4] and British Commonwealth.[5]

In 1945, the war ended in a victory for the Allies. The Soviet Union and the United States subsequently emerged as the world's two superpowers, setting the stage for the Cold War which lasted for the next 46 years. The United Nations was formed in the hope of preventing another world conflict. The acceptance of the principle of self-determination eventually accelerated decolonization movements in Asia and Africa, while Western Europe itself began moving toward integration.