The Truman Doctrine addressed the issue of communist expansion by giving financial assistance to countries who were in jeopardy of a communist take over.
After World War II, the US and Soviet Union emerged as global superpowers. Their battle for international influence and power was seen constantly. One example of this would be the policy of containment adopted by the US federal government. Containment was focused on stopping the spread of communism on a global scale.
This is why the Truman Doctrine called for $400 million of financial aid to Turkey and Greece. Both of these countries suffered damage after World War II and both were in jeopardy of being taken over by the Soviet Union.