The attempt to stop communism from spreading to Greece and Turkey after World War II was due to the Truman Doctrine.
The Truman Doctrine was a law proposed by US President Harry Truman. His goal was to stop the spread of communism (also known as containment). To ensure that Greece and Turkey (which are close to the Soviet Union) did not fall under the control of a communist nation, the US agreed to give $400 million to these countries. The goal was to help these countries recover from World War II as well as develop a political and economic alliance.