When the United States joined World War II, the Allies initially focused their military efforts on Japanese forces in the Pacific.
After Japan bombed the American fleet in Pearl Harbor, Hawaii, on December 7, 1941, the United States joined the Allies entering the WWII and was primary focused on the Japanese enterprises in the Pacific, and on gaining strategic advantage in the Pacific.
World War II was the largest armed conflict in human history. The war began with Nazi Germany's attack on Poland in September 1939.