How do realists explain the United States being a member of NATO?

A realist would explain the United States had to be a member of the North Atlantic Treaty Organization. After World War II, we were very concerned about the spread of communism. Communism had spread over much of Eastern Europe, and there were concerns it would spread to Western Europe. The United States didn’t want to take on this battle alone.
NATO was created to have a military alliance between the United States and many of the countries of Western Europe. The members of NATO were countries that didn’t have communism. By forming this military alliance, these noncommunist countries could work together, militarily if necessary, to stop the spread of communism. The creation of NATO was part of our policy of containment, which was designed to prevent communism from spreading. The fear of the spread of communism was real, which would prompt a realist to conclude that the United States had to be a part of NATO.