The Marshall Plan it can be argued was the most important event in the Cold War. With Europe devastated and in economic ruin, it was becoming increasingly susceptible to turning towards Communism. Britain, which was at least politically secure, was running out of money, and unable to support the anti-communist forces in the Grecian civil war. With a weak Britain, America became very concerned.
If Britain was unable to support her massive empire, especially in the Middle East, it too could fall to communism, threatening the increasingly important oil fields there.
Before the war the British had been the world leaders, but after the war this was impossible, the Americans would have to take their place. The Marshall Plan was the beginning of America's turn towards global security, and defined them as the worlds #1 super power.
The 12 billion dollars granted to the European nations from 1948-1952, set the precedent for American spending to contain the communist threat. On top of this was the success of the Marshall Plan achieved. It proved that communism could be contained effectively without the need to roll back the communists with nuclear weapons,proposed by loose cannons like General Douglas MacArthur
Naturally, communism and capitalism oppose each other. With the build-up of the capitalist system in Western Europe, Eastern Europe felt instantly threatened. This lead them to build themselves up and by solidifying control in the eastern bloc buffer zone. This reaction in turn threatened the West, who felt the need to build-up their own weapons and economic systems to feel secure. This one upmanship became dangerous, leading to the build up of military hardware and nuclear weapons systems that came to a head in 1962 with the Cuban Missile Crisis.
While the Truman Doctrine stated that America would support minorities in the world threatened by communism, it didn't really mean anything unless they acted upon these promises. The Marshall Plan, as President Harry Truman stated "was the other half of the walnut", that actually did something and set the precedent for America to pour money into such places as Korea, and Vietnam. Of course these events turned into hot wars once economic aid was not enough.
Socially the Marshall Plan had interesting effects upon America. At the end of World War 2, not everyone thought giving Europe 12 billion dollars was a great idea. This in turn created the need to sell the Marshall Plan to politicians and the public. Something which would be repeated time and again during the course of the Cold War, with the growth of the Military Industrial Complex. This was done by scarring the hell out of the American public by over exaggerating the communist threat, and vilifying the Russians in general. This allowed politicians like Joseph McCarthy and Richard Nixon to advance their careers artificially during the period known as the Red Scare.
In the end, the Marshall Plan was the stepping stone
to continued communist containment
around the world, the increased spending by both sides of the Cold War, and the communist scare within America.