<P> The U.S. countered with a buildup of American airpower to support ARVN defensive operations and to conduct Operation Linebacker , the first offensive bombing of North Vietnam since Rolling Thunder had been terminated in 1968 . The PAVN attacks against Huế , An Lộc , and Kon Tum were contained and the ARVN launched a counteroffensive in May to retake the lost northern provinces . On September 10 , the South Vietnamese flag once again flew over the ruins of the Citadel of Quảng Trị City , but the ARVN offensive then ran out of steam , conceding the rest of the occupied territory to the North Vietnamese . South Vietnam had countered the heaviest attack since Tet , but it was very evident that it was totally dependent on U.S. airpower for its survival . Meanwhile , the withdrawal of American troops , who numbered less than 100,000 at the beginning of the year , was continued as scheduled . By June only six infantry battalions remained . On August 12 , the last American ground combat division left the country . However , the U.S. continued to operate the base At Long Binh . Combat patrols continued there until November 11 when the U.S. handed over the base to the South Vietnamese . After this , only 24,000 American troops remained in Vietnam and President Nixon announced that they would stay there until all U.S. POW 's were freed . </P>
who was president of the united states when the last us combat troops left vietnam answers.com
Nixon