How did the United States enter World War II? The United States was not in an official state of war with Germany, Japan, or Italy when it entered World War II. The U.S. declared war on Japan after the attack on Pearl Harbor in December 1941, and on Germany, Italy and other Axis powers after December 8th 1942 when a B-25 bomber from Australia that crashed into Darwin harbor on the Northern Territory coast of Australia was confirmed to have been carrying four tons of bombs intended for Darwin's U.S. base. The German invasion of the Soviet Union in June of 1941 led to the European War which was a cold conflict in which Germany and the Soviet Union on one side, and Great Britain, France and the U.S. on the other. World War II would have ended without war if the U.S. had not entered into it when it did; this was due to a number of reasons, but mainly because there was a general fear amongst U.S. leaders that the German nation was growing too strong, and becoming too much of a threat to the free world. When the attack happened on Pearl Harbor by Japan in 1941, the U.S. immediately entered into a state of war with them. The attack was undertaken by the Japanese military at that time with no prior notice to U.S. officials, which led President Franklin D.