We all know WW2 went on from 1939-1945, but do we actually know what started it? World War began in 1939 when Germany invaded Poland and ended in 1945 with the surrender of Germany and Japan. The allied nations including Australia, Canada, Great Britain, France, Soviet Union, United States, and New Zealand defeated the Axis countries of Germany, Italy, and Japan. The United States entered the War on December 7, 1941, the day that the Japanese attacked Pearl Harbor. By the end of the War, more people were killed, more property destroyed, more people's lives were disrupted, and probably more far-reaching consequences were caused than any other war in history. World War II brought about the downfall of Western Europe as the center of world power, led to the rise of the Soviet Union, set up conditions leading to the 'Cold War,' and opened the nuclear age.