By 1969, at  the height of the war, the United States had about 543,000 troops  in Vietnam. Many of them were teenagers. The average age of  Americans fighting in Vietnam was 19.    END OF THE WAR Although Nixon increased the bombing of North  Vietnam, he began withdrawing U.S. troops. Without U.S. support,  South Vietnam's government collapsed. North Vietnam won the war in  1975. Vietnam was reunited as a Communist nation.