World War II (WWII or WW2), also known as the Second World War, was a global war. It lasted from August 1939 to November 1945, though some related conflicts in Asia began before 1939. 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 in Europe and in the Pacific, as well as in North America. It was the most widespread war in history, and directly involved more than 100 million people, from more than 30 different 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 first use of 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.
The Empire of Japan aimed to dominate East Asia and was already at war with the Republic of China in 1937, but the world war is generally said to have begun on September 1, 1939 with the invasion of Poland by Nazi Germany, resulting in declarations of war by France and Britian. Followed by subsequent declarations of war on Germany by Japan and Italy.
From late 1939 to early 1941, in a series of campaigns and treaties, Germany formed the Axis alliance with Japan and Italy, conquering or subduing much of continental Europe. They would also form a treaty with the Soviet Union called the Nazi-Soviet Pact which would remain in effect until January 1944 with the Nazi Invasion of G
Following the an offensive into central Germany, a series of Soviet defeats on the Eastern Front, the Axis powers launched an invasion of the Soviet Union, the Red Army lost the initiative and undertook strategic retreat on all fronts.
The United Kingdom and the other members of the British Commonwealth were the only major Allied forces continuing the fight against the Axis, with battles taking place in North Africa and the Horn of Africa as well as the long-running Battle of the Atlantic. In December 1941, Japan taking advantage of the war in Europe, attacked the United States and European territories in the Pacific Ocean, and quickly conquered much of the Western Pacific.