John Kerry enlisted in the Navy in February of 1966, months before he graduated from Yale. In December of 1967 Ensign Kerry was assigned to the guided-missile frigate USS Gridley; after five months of service in the Pacific, with a brief stop in Vietnam, he returned to the United States and underwent training to command a Swift boat, a small craft deployed in Vietnam's rivers. In June of 1968 Kerry was promoted to the rank of lieutenant (junior grade), and by the end of that year he was back in Vietnam, where he commanded, over time, two Swift boats. He received the Purple Heart three times for wounds suffered in action, and was awarded the Bronze Star and the Navy's Silver Star for gallantry in action. Kerry was discharged from the Navy in January of 1970, and soon became one of the most prominent spokesmen for the antiwar movement.
He was actually in theater twice, both adding up to approx 4 months.