Which countries did the United States fight against during World War II?

The United States fought against Japan, Germany, and Italy during World War II, forming the central axis of the conflict involving the Allied powers. Japan was a significant adversary in the Pacific Theater, particularly after the surprise attack on Pearl Harbor in December 1941, which prompted the U.S. to formally enter the war. Germany, led by Adolf Hitler, was the primary antagonist in Europe, initiating the conflict with its aggressive expansion and invasion of neighboring countries. Italy, under Mussolini, aligned with Germany and was involved in various military campaigns alongside them.

This choice reflects the primary aggressors against whom the United States and the Allies directed their military efforts throughout the war. The other options include countries that did not play significant roles as adversaries to the U.S. in World War II, thus differentiating why this answer is accurate and relevant to the question.