asiabuchanannom asiabuchanannom 20-04-2017 History contestada . The United States declared war on Japan in 1941. Why did it take until 1945 to defeat Japan? Japan was a more determined enemy than Germany. The Allies had to wait until Stalin's forces were available to fight in the Pacific. The Allies' island-hopping strategy proved a failure. The Allies had agreed to concentrate first on the war in Europe.