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.
They believed that all countries should have equal trading rights,they also believed that they secure the right to build a canal through central america, and that their navy was strong.
