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.
The world has breathed a collective sigh of relief after the superpowers reached an agreement ending the immediate threat of nuclear war. Russian leader Nikita Khrushchev has agreed to dismantle all Russian missiles based in Cuba and ship them back to the Soviet Union.
