The most accurate summary of the United States' involvement in World War I is:
The U.S. became involved in World War I after the Zimmermann Telegram, and sent troops to provide military reinforcements and support.
The Zimmermann Telegram, sent by Germany to Mexico in early 1917, proposed a military alliance against the United States, which, along with other factors such as unrestricted submarine warfare, prompted the U.S. to enter the war on the side of the Allies.