World War II was a global conflict 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 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 three 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, United Kingdom, and China, formed the Allied coalition, eventually defeating the Axis powers and liberating the territories occupied by Nazi forces.
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, and the prevention and mitigation of future conflicts. The war and its immediate aftermath sparked numerous uprisings. In many Sub-Saharan African nations, the colonial powers were forcibly removed, leading to a number of conflicts as new governments struggled to solidify their power. The Cold War, a period of heightened political and military tension between the Western Bloc (led by the United States) and the Eastern Bloc (led by the Soviet Union), began in the aftermath of World War II and lasted until the collapse of the Soviet Union in 1991.
World War II had a profound impact on the course of world history. The war and its aftermath marked the end of European dominance in world affairs and led to the emergence of the United States and the Soviet Union as superpowers. 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 war also had a major impact on the Holocaust, in which approximately six million Jews and other minority groups were systematically murdered by the Nazi regime and its collaborators.
The causes of World War II are complex and have been subjects of much debate, but the most prominent causes included the aggressive actions of Nazi Germany, led by dictator Adolf Hitler, and the failure of the international community to effectively address those actions. The Treaty of Versailles, which ended World War I and imposed heavy penalties on Germany, is often cited as a contributing factor to the rise of Hitler and the Nazi Party, as the harsh terms of the treaty were seen as punishment for a war that many
