World War II, or the Second World War (often abbreviated WWII or WW2), was a global military conflict lasting from 1939 to 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. Marked by significant action against civilians, including The Holocaust and the first use of nuclear weapons in warfare, it was the deadliest conflict in human history, with over seventy million casualties.
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 France and most of the countries of the British Empire and Commonwealth. Many countries were already at war by this date, such as Ethiopia and Italy in the Second Italo-Abyssinian War and China and Japan in the Second Sino-Japanese War. Many that were not initially involved joined the war later in response to events such as the German invasion of the Soviet Union and the Japanese attacks on the U.S. Pacific Fleet at Pearl Harbor and on British overseas colonies, which triggered declarations of war on Japan by the United States, the British Commonwealth, and the Netherlands.
In 1945 the war ended in an Allied victory and a changed world. While the United Nations was established to foster international cooperation and prevent future conflicts, the Soviet Union and the United States emerged as two rival superpowers, setting the stage for the Cold War which lasted for the next 46 years. Meanwhile, the acceptance of the principle of self-determination accelerated decolonization movements in Asia and Africa, while Western Europe began moving toward economic recovery and increased political integration.