What was the main goal of American combat soldiers in Vietnam?
The answer depends upon the interpretation of the question.
Explanation
As far as official US policy is concerned their main goal or objective was to help the South Vietnamese defeat the communist Vietcong and North Vietnamese army.
This was an example of a central thread of US policy since 1945, expressed through containment, the Truman Doctrine and The Domino Theory amongst others.
Another answer could be simply staying alive. Early optimism turned to pessimism and opposition, not only in the USA but also within the US army in Vietnam. By the late 60's, many of the common "grunts" we're simply staying alive. They were willing to disobey orders, particularly from inexperienced officers, and seek solace in drugs which were freely available.
The fact that one in 5 Americans were killed by their own side speaks volumes about the organisation of US involvement and the extent to which American soldiers could place faith in their commanders
