The US entered the war with their declaration on 6 April, 1917. The war technically ended (the armistice) on November 11, 1918. They were at war for one year, six months and five days.
It depends on what you consider involvement. The United States declared neutrality but showed clear favoritism to England and France rather than Germany in trade terms. As for any actual declared...
the single most important reason the U.S. got involved in the war was because they felt that their position as a neutral country had been violated because a German U-boat had sunk the Lusitania, an...