World War II, also known as the Second World War, 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 centres (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 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, United Kingdom, and China, formed the Allied coalition, which defeated the Axis powers on the Eastern Front and in the Pacific War. 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. The Japanese, facing an invasion of Manchuria by the Soviets, a naval blockade, and the prospect of a costly invasion of the Japanese Home Islands, agreed to a unconditional surrender on 15 August 1945, ending the war in Asia and cementing the total victory of the Allies.
The dropping of the atomic bombs on Hiroshima and Nagasaki in August 1945 by the United States was a turning point in the war, leading to Japan's surrender and the end of the war. The United States and the Soviet Union emerged as the world's two superpowers, setting the stage for the Cold War, which lasted for the next 46 years. Meanwhile, 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 global economic development.
World War II had a profound impact on the world, including the formation of the United Nations and the emergence of the United States and the Soviet Union as superpowers. It also led to major technological advancements, including the first nuclear weapons and the jet engine, and was a driving force behind the development of the rocket technology that would later be used to send humans to the moon.
The war also had a significant impact on the home front, with governments implementing policies such as rationing and conscription to mobilize their populations for the war effort. Civilian populations also suffered greatly, with bombings and other attacks on cities leading to widespread destruction and loss of life.
Overall, World War II was a devastating conflict that had far-reaching consequences for the world. It is important to remember and learn from the lessons of the war