2 It was the beginning of American policy for the cold war. This policy also had another name "Containment". Communism had to be contained so that it should and would not spread any further. The Russians and their Communist ideals were ruffling the Americans and their capitalistic feathers. The Americans assumed that as soon as one country became Communist, the neighboring countries would soon fall prey to Communism like dominoes. Thus it got its name- The Domino Theory. Most significantly the Truman Doctrine also committed the USA to play a big part in world affairs. The USA, not the UN, would be the world's policeman.
"I believe that it must be the policy of the United States to support free peoples who are resisting attempted subjugation by armed majorities or by outside pressures. The free peoples of the world look to us for support in maintaining those freedoms. If we falter in our leadership, we may endanger the peace of the world." 3 .
The Soviets had their own response, with the Council for Mutual Economic Assistance (COMECON), to assist and coordinate the economical development of its members.
For every move that America made, the Soviet Union made as well, as a foil to the capitalist and democratic superpower on the other side of the continent. Continued Russian attempts to align more countries on its side made the United States (primarily) set up the North Atlantic Treaty Organization (NATO) in 1949. Its primary function was that if a fellow member were attacked, the rest of the members would strike back at the hostile nation- collective security. This was a thinly veiled warning to Russia, not to try and spread its sphere of influence militaristically. NATO's policy of defending a fellow member if attacked still stands today though it has not being used, although it is might be used if there is a war, America's and the rest of the world waging war against terrorism. The defense of Western Europe came to rest solely in NATO.