the domino theory and truman doctrine
The foreign policy that supported the US involvement in Vietnam was based off of the Truman Doctrine. The Truman Doctrine allowed the US to intervene in foreign conflicts that could result in a communist overthrow. The idea was to keep communism contained, for the US feared the spreading power of the communist rule and that of Soviet Russia. Sometime after the Truman Doctrine was put into effect, the idea of the Domino Theory came to be known. The Domino Theory stated that if one country fell to communism, then many nearby countries would also fall to communist rule. These two ideas, in combination, were what the US used to justify its involvement in Vietnam.