World War II was a global war that lasted from 1939 to 1945 and 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. It was the most widespread war in history, and directly involved more than 100 million people from more than 30 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 Holocaust (in which approximately 11 million people were killed) and the strategic bombing of industrial and population centers (in which approximately one million people were killed), it resulted in 50 million to over 70 million fatalities.
The war in Europe began with the invasion of Poland by Nazi Germany and the Soviet Union, followed by the British and French declaration of war on Germany in September 1939. From late 1939 to early 1941, in a series of campaigns and treaties, Germany conquered or controlled much of continental Europe, and formed the Axis alliance with Italy and Japan. Under the Molotov–Ribbentrop Pact, Germany and the Soviet Union partitioned and annexed territories of their European neighbours, including Poland, Finland, and the Baltic states.
In June 1941, Germany turned on the Soviets, opening the largest and the deadliest theatre of war in history. Nazi Germany acquired additional territories in eastern Europe, invaded the Soviet Union, and embarked on a massive campaign of extermination and enslavement, eventually committing the genocide of over 3 million Soviet and Polish Jews, as well as various Romani peoples, gay people, disabled people, priests, political opponents, and others deemed "unworthy of life" by the Nazi regime. In response, the Soviet Union, along with the United States, China, and several other countries, formed the Allied coalition, making substantial contributions in the Pacific War and ultimately defeating the Axis.
The war in Europe concluded with the capture of Berlin by Soviet and Polish forces, and the subsequent German unconditional surrender on 8 May 1945. Following Germany's defeat, the Allied powers partitioned Berlin and Germany's territories, establishing two separate German states: West Germany, formed from the American, British, and French occupation zones, and East Germany, formed from the Soviet occupation zone.
The war in the Pacific ended on 15 August 1945, when Japan announced its unconditional surrender to the Allies, following the atomic bombings of Hiroshima and Nagasaki and the Soviet invasion of Manchuria. The dropping of the atomic bombs, along with Germany's surrender, marked the end of World War II, resulting in the defeat and decline of the Axis powers and the Allied victory.
The United Nations (UN), formed in the aftermath of the war, and including many of the victorious powers, became the first international organization to address issues of global concern, including decolonization, peace and security, and human rights. The victorious great powers—China, France, the Soviet Union, the United Kingdom, and the United States—became the permanent members of the United Nations Security Council. The Soviet Union and the United States emerged as rival superpowers, setting the stage for the Cold War, which lasted for the next 46 years.
