The United States worried that if a country became communist or had a communist takeover, there would quickly be more countries that would fall to communism, like dominos. That's why this idea is called the Domino Theory. It was mainly a Cold War policy, and was the reason the U.S. got involved with Vietnam.
The Domino Theory was and still is popular, but if your question is asking about a more recent policy, please let me know and ill find some more information for you.
