The long American tradition of isolationism made many Americans want to withdraw from international affairs after WWII. They did not want to get tied to any permanent alliances with any countries in places (like Western Europe) that did not have any direct and obvious relevance for US security.
This came into tension with the needs that were brought on by the Cold War. The Cold War made it important for the US to be dedicated to relationships with the countries of Western Europe. They needed to form long-lasting relationships with those countries so that they could work together to hold back communism.
In this way, the needs of the Cold War came into tension with the idea of isolationism that had long been a major part of US foreign policy.