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 reflects the progression of U.S. involvement, starting from support for the South Vietnamese government with aid and military advisors, escalating to direct military engagement after incidents such as the Gulf of Tonkin incident, where U.S. warships were reportedly attacked.