The Truman Doctrine featured the belief of containing Communism at all costs and being a focal point of U.S. Foreign policy.  At the end of the Second World War, meetings between American, British and Soviet leaders revealed a very deep division between the makeup and composition of Postwar Europe.  Stalin sought to create Soviet spheres of nations as a Communist block of nations was being envisioned by the Soviet leader.  The perception was that this could end up being a defining paradigm around the world and in Europe.  To this end, the belief of the domino theory of nations in their relationship with Communism became advanced, suggesting that once a nation in a particular region "went Communist," all of the nations in the region would follow the same pattern  Containment, as advocated through the Truman Doctrine, became the only way that the United States saw in being able to deal with such a paradigm and execute foreign policy.

The Truman Doctrine went far beyond what most people think, it was not simply some plan about aiding countries facing communist insurgencies.  The doctrine did involve military aid, but also life-enhancing programs such as engineering projects, agricultural projects, health care, the full range of aid programs.  It also did involve the Marshall Plan, but the most important long-range part of the plan was the doctrine of Containment.

This did not involve only aiding countries against invasion (such as the Greek-Albanian War and the Korean War), insurgencies (a myriad of wars from the Malaysia Insurgency on) and situations involving both (such as Vietnam and the Soviet-Afghan War).  The long term plan was to keep the wars as small and contained as possible, which was most difficult in Vietnam, and to gradually economically wear down the USSR.  Essentially, from 1947 through the 1980s the "World Revolution" proceeded at a pace and a cost which drove the Soviet economy into a state of disaster.  The Afghan war was the final nail in the coffin, so to speak.

By the mid-70s the probability of war between the US and USSR was really low, and the proxy wars seemed to be going fairly well for the Soviets.  It was a trap, basically.  In the '80s the process in Central America was to adjust our aid to just above what the communists could deal with, prompting them to pour in more men, material and materiel to top that, then we would add a little more, all the time using our economic might, international credit and diplomacy to keep a lid on the possible size of the wars.  Same in Africa, etc.  With the increase in costs, the Russian economy began to falter.  The French destroyed unknown millions of dollars worth of tanks and aircraft Libya still owed the Soviets for, as an example.  The Israeli invasion of Lebanon seized enough Soviet weapons to arm several divisions, hundreds of thousands of soldiers, an indication that the USSR was stockpiling weapons in the Middle East in the hopes of an eventual invasion; the PLO could never raise a fraction of the troops to use them.  The Syrian Army, which had invaded Lebanon earlier and held the eastern part of the country, had so many Soviet "advisers" that some front-line units were 10% Russian.  If you have a fifty-thousand dollar missile and use it to kill a million dollar tank, you're winning.  If you convince your enemy to use billions of dollars for weapons easily destroyed in the hands of third parties, or never even used, that's even better.

With the war in Afghanistan, the Soviet Union reached its breaking point.  With the collapse of morale in the army and faith in the government at home, combined with the economic strain of the war, the end came.  By the mid-80s the USSR was doomed; two books came out at the time, one written by Richard Nixon, declaring that for all practical purposes the Cold War was over, and the Soviets had lost.  The final collapse was simply a matter of the politicians realizing the truth and scrambling for whatever future they could set up for themselves and their country.

And all of this, which so many like to pretend was some magical effect of Reaganism, was accomplished by following the principles of Containment, and the Truman Doctrine.

Preventing the spread of communism in other countries (especially Greece and Turkey) was very important and President Truman was very passionate about keeping communism out. This is when the Truman Doctrine was adopted. Here is what he had to say about it:

"I believe that it must be the policy of the United States to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressures. I believe that we must assist free peoples to work out their own destinies in their own way. I believe that our help should be primarily through economic and financial aid, which is essential to economic stability and orderly political processes.”

The Truman Doctrine was followed by the Marshall Plan (Economic Cooperation Act of 1948). The purpose of this act was to assist countries, including Turkey, avoid economic devastation caused by the war.

The Truman Doctrine was President Harry S. Truman's policy with respect to communism in the early days after World War II.  He announced this in a speech in March of 1947. The basic idea of the doctrine was that the United States would give help to any "free peoples" who were in danger of being forced into communism.

In other words, Truman was saying that any free country that had a communist insurgency going on could get support from the US.  The doctrine was applied first to Turkey and Greece.  This doctrine was the start of the policy of containment.