A major issue throughout the Cold War was the 'Iron Curtain'. The West generally argued that Eastern Europe were puppet states controlled by the USSR. Stalin believed the same for the rest of Europe.

Following the Second World War, much of Europe was in tatters. The US responded by offering financial help under the Marshall Plan, named after US Secretary of State George Marshall. The US made a take it or leave offer to all European countries, including the USSR, in need of financial assistance. Many countries took this offer (Britain only finished repaying the loan in 2006). Stalin took this as Truman trying to get influence in Eastern Europe.

Stalin was right. Truman and Marshall knew offering a loan would guarantee allegiance and knew Stalin would reject it. They hoped it would mean many Eastern European actions would turn their back on the Soviet Union for US assistance. Marxist economist Jeno Varga described the Marshall plan as a dagger pointed at Moscow.

It can be argued that with this move from the US to exert influence over Europe forced Stalin to make a move. Remember, Stalin began the policy of 'Socialism in One Country' and claimed Eastern Europe was a 'buffer zone' to protect the USSR.

So it may be said the Marshall plan could be described as the first move of the Cold War.

Source: Robert Service, Stalin: A Biography, (London: MacMillan, 2004), page 504.

We seemed to love the Domino Theory (http://en.wikipedia.org/wiki/Domino_theory) That is, the US thought that if we let communism take root, it would spread to neighboring countries. It is this thought that allowed for the Korean and Vietnam wars. However, the US did not understand that just because a country was/wanted to be communist, that didn't mean they would be the USSR's lapdog. Take Vietnam for example. The communist movement there was much more nationalistic in nature. It was a movement to gain sovereignty for their own country.