Entry of the United States

A long stretch of American isolationism left the United States reluctant to involve itself with what was popularly conceived as a European dispute.
Early in 1917 Germany resumed its policy of unrestricted submarine warfare. This, combined with public indignation over the Zimmerman telegram, led to a final break of relations with the Central Powers. President Woodrow Wilson requested that the U.S. Congress declare war, which it did on April 6, 1917 (see: Woodrow Wilson declares war on Germany on Wikisource). The Senate approved the war resolution 82-6, the House with 373-50.
Although the American contribution to the war was important, particularly in terms of the threat posed by increased US presence in Europe, the United States was never formally a member of the Allies, but an "Associated Power".
The United States Army and the National Guard had mobilized in 1916 to pursue the Mexican "bandit" Pancho Villa, which helped speed up the mobilization. The United States Navy was able to send a battleship group to Scapa Flow to join with the British Grand Fleet, and a number of destroyers to Queenstown, Ireland, to help guard convoys. However, it would be some time before the United States forces would be able to contribute significant manpower to the Western and Italian fronts.
The British and French insisted that the United States emphasize sending infantry to reinforce the line. Throughout the war, the American forces were short of their own artillery, aviation, and engineering units. However, General John J. Pershing, American Expeditionary Force commander, resisted breaking up American units and using them as reinforcements for British and French units, as suggested by the Allies.
The reasons the United States got involved in the war are numerous and much-debated. In 1934, the US government created the Nye Committee to investigate the matter. In 1936, the committee reported that between 1915 and April 1917, the US loaned Germany 27 million dollars ($27,000,000, or $470,000,000 adjusted for inflation in 2005 dollars). In the same period, the US loaned Britain and its allies 2.3 billion dollars ($2,300,000,000 or $40,000,000,000 adjusted for inflation in 2005 dollars), or about 85 times as much. They concluded that the US entered the war because it was in its commercial interest for Britain not to lose.