The United States declared war on Japan the day after the Japanese attacked the American naval base at Pearl Harbour (December 7, 1941). In response, Italy and Germany, Japan's allies, declared war on the United States, making the U.S. a full partner in the war against Germany. However, back in March of that year, in the context of the war in the Atlantic, American naval vessels had begun patrolling maritime routes in the western Atlantic in order to relieve the pressure on the British navy. In September 1941, a German submarine attacked the American destroyer Greer; and in October, a German submarine sank the American vessel Reuben James. President Roosevelt ordered his fleet to carry out a reprisal attack, thus bringing the U.S. into the campaign at that early stage, albeit indirectly.
The official and full participation of the U.S. in the war against Germany and its allies would become one of the main factors in Germany's defeat. The other two Axis countries, Bulgaria and Hungary, added their names to the declaration of war on December 13.
