World War II (WWII) was the largest war in the history of the world. It involved the most people, and the most countries of any war ever. It also ended up killing more people than any other war ever did. This war is generally said to have started in September of 1939 and to have ended in August of 1945.
World War II pitted two coalitions of allies against one another. On one side was the Axis, whose main powers were Germany, Japan, and Italy. On the other side were the Allies, whose main powers were Britain, France, the Soviet Union and the United States. The war was generally about the desire for more power on the part of the Axis.
The war was generally separated into two main theaters. These were the European theater and the Pacific theater. War began in Europe first, when Germany invaded Poland in September of 1939. The war in Europe originally involved only Britain and France against Germany, with Italy somewhat involved. The Soviet Union got involved in June of 1941 when it was invaded by Germany. The war in the Pacific started on December 7, 1941 when Japan attacked the US at Pearl Harbor. This theater was mainly an American-Japanese war. The US also got heavily involved in the war in Europe.
The war ended with the utter defeat of the Axis. Italy had surrendered separately relatively early in the war but Germany and Japan fought on until May and August of 1945,respectively.