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World War II was the most widespread and costly war in history. Fighting took place across much of the world, from Norway to the island of New Guinea, and an estimated 60 million people were killed or died from war-related famine. On one side of the war were the United Kingdom and the British Commonwealth (led by Winston Churchill), France, the United States (led by Franklin Delano Roosevelt), the Soviet Union (led by Joseph Stalin) and China, with several smaller countries. This side is often referred to as "the Allies". On the other side were Germany (led by Adolf Hitler), Italy (led by Benito Mussolini) and Japan (led by Hideki Tojo), with several smaller countries, in a group called "the Axis". The main combat areas were Europe, East Asia and the Pacific Ocean.
The exact date of the outbreak of the war has not been agreed. The most commonly accepted date is September 1, 1939, when Germany invaded Poland, with Britain and France declaring war on Germany two days later. Others cite 7 July 1937 as the date of the outbreak of the war, when Japan invaded China and others then marked March 1939 and the arrival of Hitler's forces in Prague, Czechoslovakia. It was only in 1941 that the war became truly global. In June of that year, Germany invaded the USSR, while in December Japan attacked the US navy at Pearl Harbor. Shortly afterwards, Germany declared war on the United States in support of Japan. The war is usually thought to have ended on 2 September 1945, when Japan surrendered to the Allies.
Military technology had advanced significantly since World War I, and aircraft had become much more important. Air raids on enemy cities became an important element in both sides' strategy, and this was one of the reasons that the total losses among the civilian population were much higher in this war than in the First World War. The war ended when America dropped atomic bombs on the cities of Hiroshima and Nagasaki in Japan.
During this war millions of people were killed in German concentration and extermination camps. Some 6 million Jews were killed in the Holocaust, as well as smaller but significant numbers from many other groups. By the end of the war, the United States and the USSR were the two great powers, setting a pattern that lasted for several decades. Germany was divided into two, which continued until 1990. Although the former imperial powers such as the United Kingdom and France were on the winning side, they were significantly weakened by the war.
