America only entered World War I in Spring, 1917 when Germany resumed unrestricted submarine warfare. Prior to that time, the U.S. had attempted to maintain neutrality. At the time the U.S. entered the war, both the Allied and Central Powers were exhausted and locked in a stalemate. With fresh troops and the strength of its economy behind it, the U.S. entry into the War played a key role in the collapse of German lines. Primary American commander in World War I was General John J. "Black Jack" Pershing; who presumably said upon landing in France, "Lafayette, we are here." The war ended November, 1918; so U.S.participation was not that lengthy, but was important.
The U.S. danced around neutrality originally in World War II; although it was more involved than the government would like to admit. The U.S. furnished Britain with "overaged" naval destroyers and also stopped shipments of oil and scrap metal to Japan before it became actively involved. Japan had already determined that war with the U.S. was inevitable and planned the attack on Pearl Harbor to destroy the U.S. Pacific Fleet.
The U.S. entered the war after the Japanese bombed the fleet at Pearl Harbor on December 7, 1941. This was one day after the German advance on Stalingrad was halted on December 6. Two days after the U.S. declared war on Japan, Germany and Italy declared war on the U.S. American role in World War II was much more decisive than in World War I. Dwight D. Eisenhower, later president of the United States, was Supreme Commander of the European Theater of Operations and personally directed the D-Day invasion of Normandy. In the Pacific Theater, General Douglas McArthur commanded ground forces and Admiral Chester Nimitz commanded Naval forces. The war ended after an American plane dropped an Atomic bomb on Hiroshima and later Nagasaki in August, 1945.
The United States attempted to maintain a position of neutrality in both wars. This was until military maneuvers caused the United States to enter into the conflict.
The position of the U.S. in World War I was neutrality; however, the blockades of British ports caused issue. The Germans has used unrestricted submarine warfare to sink the Lusitania, a passenger ship, several Americans. The Americans changed their view of Germany and the Central Powers after this act. The Germans temporarily stopped the use of submarine warfare until they decided that the British would starve before the United States could help so they began to attack ships coming into Britain. The United States entered the war on April 1917. By 1918 large numbers of American troops arrive in Europe and Germany launched its last great offensive at the Battle of the Marne. After this turning point U.S. and allied troops began to push back the Germans until their surrender in November 1918.
World War II began with Germany invasion of Poland in 1939. Germany made such rapid progress through Europe that Great Britain asked the United States for assistance. Still practicing a policy of isolationism, the old alliance was strong and so the United States supplied food, ships, planes and weapons under the Lend-Lease plan. The United States entered the war after the Japanese bombed Pearl Harbor on December 7, 1941. The tide of the war began to turn in 1942 when the United States and British troops forced the Germans and Italians to surrender in North Africa. At the same time the German troops were surrounded in Stalingrad by Soviet forces. The Battle of Midway in the Pacific destroyed four Japanese aircraft carriers thus establishing United States naval superiority in the Pacific. Beginning in 1943 the Axis powers began a defensive war and the Allies made significant victories from that moment forward until the end of in 1945.