Truman Doctrine
Designed by former US President Harry Truman, it pledged US support to all countries that were trying to fend off communist threats. It was applied most effectively to Western European countries and was a part of the US's larger "containment policy" which was designed to prevent the spread of communism.
Links
http://www.luminet.net/~tgort/truman.htm
http://www.cyberessays.com/History/160.htm