The most accurate summary of the United States' involvement in World War I is:
Initially, the U.S. was politically neutral and was involved only economically; however, once the U.S. entered the war its military involvement helped force an armistice.
This statement captures the initial neutrality of the U.S., its significant economic support to the Allies before officially entering the war in 1917, and the influential role of American military forces in contributing to the eventual armistice and Germany's surrender in 1918.