The Cold War/The Truman Doctrine: Beginning of Proxy War
As the USA realized that it was being faced with the rapid spread of the new threat of communism, they decided to tackle that "threat" headlong and face-to-face, by supporting non-communist governments and armed groups fighting against the spread of communism to their nations. This new and more aggressive attitude of "containing" communism within it's own borders was basically the Truman Doctrine
Containment in the making 
When American President, Franklin Roosevalt died suddenly just days before the end of World War Two, a new president Harry S. Truman was elected to replace him. Truman was, as you would put it, a fierce anti-communist who believed that the USSR wanted communism to dominate the world and this put all the capitalist countries of the West and the wider world, and Western interests at danger.
In February 1947 he was faced with two very serious problems. In Greece, communist insurgents were fighting a guerilla war against the Greek Royalist government, and trying to overthrow it. Britain had 40,000 troops stationed there to assist the government in fighting the communists. But all of a sudden, British foreign secretary Ernest Bevin said that the country could no longer afford to keep it's troops there because of the current economic situation back home and were to withdraw them. Meanwhile on the Turkish border, Soviet troops were massing heavily on the border and Stalin was waiting for the right time to invade.
Truman was under enormous pressure. He knew that he had to take a firm stand against the communist onslaught, otherwise this would lead to a dominoe effect with country after country getting their governments toppled and replaced by a communist "regime" and soon the entire world would be in the hands of the communists.
On March 12th 1947, he made a dramatic and famous speech to Congress and it was this quote that signalled the start of the Truman Doctrine:
|“||I believe 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||”|
This meant that Truman believed U.S. policy should support non-communist countries which were coming under attack internally by pro-communist movements or being faced with the threat of invasion by communist countries. From a military perspective, you could almost certainly say that this was the beginning of proxy war against the Soviet Union.
He then continued his speech, and did a stinging attack on the communist way of life, saying that it was "evil", and "denied human rights" and "living standards were low".
Congress listened in uneasy silence. One member commented soon after that Truman had "scared the hell out of them". But nevertheless, they unanimously voted for a sum of $400 million to Greece and Turkey, and this money helped the Greek government in defeating the communist rebels and served as a warning sign to the USSR to keep out of Turkey
The Truman Doctrine greatly infuenced American Cold War politics and relations with the Soviet Union, for the next 40 or so years. But it has also influenced American foreign politics today and was one of the major factors in turning the USA from isolationist in foreign policy to moderatly interventionist in global affairs.