The main reason for united states military involvement in vietnam in the early 1960s was to

To stop communism.

Explanation:

After the assassination of President Kennedy in 1963, Lyndon Johnson became the new President of the USA. He referred to the domino theory, which he meant that if Vietnam became a communist country, other countries in South Asia might follow its example, much like one domino sets off another until they have all fallen down. Johnson decided to send American soldiers to the war in Vietnam to prevent this from happening.

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