The United States tried to limit North Vietnam's power because North Vietnam was communist.
At this time in history, the US had a policy of containment. This policy was focused on stopping the spread of communism on a global scale. The US was worried about the spread of communism, as it would result in the increased power and influence of the Soviet Union. At this time, the US and Soviet Union were in the midst of the Cold War and were constantly on the verge of warfare.