World War II was a global war that lasted from 1939 to 1945 and involved the 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 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 the other Allies, eventually defeated the Axis powers and liberated Europe.
The drop of the atomic bombs on Hiroshima and Nagasaki in August 1945 marked the end of World War II, as the Japanese surrendered to the Allies. The United States, the United Kingdom, and the Soviet Union emerged as the world's three dominant powers, and the United Nations, 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, and the prevention and mitigation of future conflicts. The Cold War, which began in 1947 and lasted until the collapse of the Soviet Union in 1991, was largely a continuation of the Western and Eastern conflict and resulted in the emergence of the Western Bloc and the Eastern Bloc, with the United States and its allies, including the UK, facing off against the Soviet Union and its allies.
The war had a profound impact on the course of world history. The United Nations, 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, and the prevention and mitigation of future conflicts. The legacy of the war and the ensuing Cold War shaped much of the second half of the 20th century, and continues to influence contemporary international relations.