The most accurate summary of the United States' involvement and military role 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.
While other events, such as the sinking of ships and the broader context of U.S. economic support, played roles in the lead-up to U.S. involvement, the Zimmermann Telegram is generally considered a key factor that prompted the U.S. to enter the war. The U.S. formally declared war on Germany on April 6, 1917, and subsequently sent troops to the front lines.