What were the causes of US entry into the Vietnam War?

The United States' entry into the Vietnam War was influenced by a combination of factors that included ideological, geopolitical, and domestic considerations:

1. Domino Theory: The US government subscribed to the Domino Theory, the idea that if one country in a region came under the influence of communism, then the surrounding countries would also fall like a series of dominoes. This was a pivotal part of the Cold War ideology where the US sought to contain the spread of communism around the world.

2. Containment Policy: To prevent the spread of communism, the United States adopted a containment policy outlined by George Kennan in 1947. This policy was central to U.S. foreign affairs and held that communism needed to be contained within its current borders to prevent further Soviet influence.

3. Support for South Vietnam: The U.S saw South Vietnam as a bulwark against the spread of communism in Southeast Asia. President Diem's government, struggling to maintain control against the Communist-backed Viet Cong insurgency, requested assistance from the US.

4. Gulf of Tonkin Incident: An alleged attack on U.S. naval vessels by North Vietnamese PT boats in the Gulf of Tonkin in August 1964 provided President Lyndon B. Johnson with the justification to escalate U.S. military involvement. The subsequent Gulf of Tonkin Resolution granted him the authority to use military force without a formal declaration of war.

5. Fear of Losing American Credibility: There was a strong belief that failure to defend South Vietnam would damage U.S. credibility with its allies and could embolden the Soviet Union and communist movements worldwide.

6. Presidential Commitments: Presidents Eisenhower, Kennedy, and Johnson had all made public and private commitments to defend South Vietnam from communism. Escalating involvement was partly an extension of these commitments.

7. Cold War Context: The broad context of the Cold War era, with its intense rivalry between the U.S. and the USSR, exerted pressure on the U.S. to confront perceived Soviet expansion wherever it occurred.