The most accurate description of the sequence of events that led to U.S. involvement in Vietnam is:
The United States first sent only aid and military advisors to assist the South Vietnamese in their struggle; however, after a U.S. warship was torpedoed, the U.S. sent troops.
This statement reflects the escalation of U.S. involvement, starting with support through advisors and aid, and leading to increased military engagement following incidents like the Gulf of Tonkin incident in 1964.