The U.S. was following the policy of containment of communism established by the Truman Doctrine. North Vietnam, led by Ho Chi Minh, invaded and killed thousands of South Vietnamese. The United States again took responsibility, as a super power, to stop the spread of Communism into South Vietnam, in order to stop human rights violations and to protect South Vietnam's right to self determination (Nash).
The stark difference in how officials gain office
President Johnson on why the United States intervened
The United States went to protect the sovereign rights of Vietnamese people