<P> The military history of the United States in World War II covers the war against Germany, Italy, Japan and starting with the 7 December 1941 attack on Pearl Harbor . During the first two years of World War II, the United States had maintained formal neutrality as made officially in the Quarantine Speech delivered by U.S. President Franklin D. Roosevelt in 1937, while supplying Britain, the Soviet Union, and China with war material through the Lend - Lease Act which was signed into law on 11 March 1941, as well as deploying the U.S. military to replace the British invasion forces in Iceland . In the Pacific Theater, there was unofficial early U.S. combat activity such as the Flying Tigers . The U.S. economic sanctions on Japan, as part of the effort to deter Japanese military aggression in Asia and the Pacific, was a major cause of the Japanese attack on Pearl Harbor . During the war, over 16 million Americans served in the United States Armed Forces, with 405,399 killed in action and 671,278 wounded . There were also 130,201 American prisoners of war, of whom 116,129 returned home after the war . Key civilian advisors to President Roosevelt included Secretary of War Henry L. Stimson, who mobilized the nation's industries and induction centers to supply the Army, commanded by General George Marshall and the Army Air Forces under General Hap Arnold . The Navy, led by Secretary of the Navy Frank Knox and Admiral Ernest King, proved more autonomous . Overall priorities were set by Roosevelt and the Joint Chiefs of Staff, chaired by William Leahy . Highest priority went to the defeat of Germany in Europe, but first the war against Japan in the Pacific was more urgent after the sinking of the main battleship fleet at Pearl Harbor . </P> <P> Admiral King put Admiral Chester W. Nimitz, based in Hawaii, in charge of the Pacific War against Japan . The result was a series of some of the most famous naval battles in history . The Imperial Japanese Navy had the advantage, taking the Philippines as well as British and Dutch possessions, and threatening Australia but in June 1942, its main carriers were sunk during the Battle of Midway, and the Americans seized the initiative . The Pacific War became one of island hopping, so as to move air bases closer and closer to Japan . The Army, based in Australia under General Douglas MacArthur, steadily advanced across New Guinea to the Philippines, with plans to invade the Japanese home islands in late 1945 . With its merchant fleet sunk by American submarines, Japan ran short of aviation gasoline and fuel oil, as the U.S. Navy in June 1944 captured islands within bombing range of the Japanese home islands . Strategic bombing directed by General Curtis Lemay destroyed all the major Japanese cities, as the U.S. captured Okinawa after heavy losses in spring 1945 . With the atomic bombings of Hiroshima and Nagasaki and an invasion and Soviet intervention imminent, Japan surrendered . </P> <P> The war against Germany involved aid to Britain, her allies, and the Soviet Union, with the U.S. supplying munitions until it could ready an invasion force . U.S. forces were first tested to a limited degree in the North African Campaign and then employed more significantly with British Forces in Italy in 1943--45, where U.S. forces, representing about a third of the Allied forces deployed, bogged down after Italy surrendered and the Germans took over . Finally the main invasion of France took place in June 1944, under General Dwight D. Eisenhower . Meanwhile, the U.S. Army Air Forces and the British Royal Air Force engaged in the area bombardment of German cities and systematically targeted German transportation links and synthetic oil plants, as it knocked out what was left of the Luftwaffe post Battle of Britain in 1944 . With the Soviets unstoppable in the east, and the Allies unstoppable in the west, Germany was squeezed to death . Berlin fell to the Soviets in May 1945, and with Adolf Hitler dead, the Germans surrendered . </P> <P> The military effort was strongly supported by civilians on the home front, who provided the military personnel, the munitions, the money, and the morale to fight the war to victory . World War II cost the United States an estimated $341 billion in 1945 dollars--equivalent to 74% of America's GDP and expenditures during the war . In 2015 dollars, the war cost over $4.5 trillion . </P>

When did the us start fighting germany in ww2