World War II, also WWII, or the Second World War, was a global military conflict that took place between 1939 and 1945. It was the largest and deadliest war in history.

Even though Japan had been fighting in China since 1937, the conventional view is that the war began on September 1, 1939, when Nazi Germany invaded Poland. Within two days the United Kingdom and France declared war on Germany, although the only European battles remained in Poland. Pursuant to a then-secret provision of its non-aggression Molotov-Ribbentrop Pact, the Soviet Union joined with Germany on September 17, 1939, to conquer Poland and to divide Eastern Europe.

The Allies were initially made up of Poland, the British Empire, France, and others. In May, 1940, Germany invaded western Europe. Six weeks later, France surrendered to Germany. Three months after that, Germany, Italy, and Japan signed a mutual defense agreement, the Tripartite Pact, and were known as the Axis Powers. Then, nine months later, in June 1941, while still battling Britain, Germany betrayed and invaded its partner, the Soviet Union, forcing the Soviets into the Allied camp (although they still abided by their non-aggression treaty with Japan). In December 1941, Japan attacked the United States bringing it too into the war on the Allied side. China also joined the Allies, as eventually did most of the rest of the world. By the beginning of 1942, the major combatants were aligned as follows: the British Commonwealth, the United States, and the Soviet Union were fighting Germany and Italy; and the British Commonwealth, China, and the United States were fighting Japan. From then through August 1945, battles raged across all of Europe, in the North Atlantic Ocean, across North Africa, throughout Southeast Asia, throughout China, across the Pacific Ocean and in the air over Japan.

Italy surrendered in September 1943; Germany surrendered in May 1945. The surrender of Japan marked the end of the war, on September 2, 1945.

It is possible that around 62 million people died in the war; estimates vary greatly. About 60% of all casualties were civilians, who died as a result of disease, starvation, genocide, and aerial bombing. The former Soviet Union and China suffered the most casualties. Estimates place deaths in the Soviet Union at around 23 million, while China suffered about 10 million. Poland suffered the most deaths in proportion to its population of any country, losing approximately 5.6 million out of a pre-war population of 34.8 million (16%).

After World War II, Europe was informally split into western and Soviet spheres of influence. Western Europe later aligned as North Atlantic Treaty Organization (NATO) and Eastern Europe as the Warsaw Pact. There was a shift in power from Western Europe and the British Empire to the two new superpowers, the United States and the Soviet Union. These two rivals would later face off in the Cold War. In Asia, the defeat of Japan led to its democratization. China's civil war continued through and after the war, resulting eventually in the establishment of the People's Republic of China. The former colonies of the European powers began their road to independence.