Admittedly, the first view of Stalin, as an imperialist leader, may be twisted. The Russians claim, and have always claimed, that Stalin's motives were purely defensive. Stalin wished to create a buffer zone of Communist states around him to protect Soviet Russia from the capitalist West. In this sense, his moves were not aggressive at all -- they were truly defensive moves to protect the Soviet system. His suspicions of Western opposition were not unfounded: the British and U.S. intervention in the Russian Civil War were still fresh in Stalin's memory when he took power. Furthermore, Stalin was bitter because he was not informed of U.S. nuclear capabilities until shortly before the atomic bomb was dropped on Hiroshima. Compounding tensions was the fact that Stalin's request that Russia be allowed to participate in the occupation of Japan was denied, even though Russia had declared war on Japan on 8th August had been responsible for occupying south Sakhalin as agreed to at Yalta. This failure to be included in the Western world's politics created an even deeper crack between the two superpowers.
From this point, the Cold War truly becomes a chain reaction. In March of 1946, Churchill presented his 'Iron Curtain' speech at Fulton, Missouri, in response to the spread of communism in Eastern Europe. He called for a western alliance to combat the threat. Stalin's response was hostile: rather than trying to negotiate a peaceful settlement, Stalin continued to tighten his grip on Eastern Europe. Communist governments were installed in every area of Eastern Europe (barring Czechoslovakia) by the end of 1947.
These governments were implemented by revolutionary tactics: elections were rigged, non-communist members of the governments were expelled, with many being arrested or executed, and eventually, Stalin dissolved all non-communist political parties. Stalin began to implement a reign of terror using the Russian Army and his secret police force. Moreover, Stalin had increased his influence in the Russian zone of Germany as if it belonged to Russia. He allowed only the communist party and drained the area of its vital resources.
The West reacted. It appeared to them that Russia's attitude went against all of the promises that Stalin had made at Yalta -- namely, that Stalin would permit free elections in the eastern European states. Russia argued that it needed to maintain a sphere of influence in the area for security reasons: to this, even Churchill agreed in 1944. Further, Russia argued that the areas had never had democratic governments, and that a communist system would allow these 'backward countries' to progress and flourish. Stalin's policy of expansion worried the West: in response, the West introduced the Truman Doctrine and the Marshall Plan, both of which wanted to arrest the spread of communism.
Perhaps the most aggressive move that Stalin made, however, was the takeover of Czechoslovakia in February 1948. Several key issues arose in this conflict. First, the U.S. felt alienated when Czechoslovakia rejected Marshall Aid, which the U.S. blamed on the influence of the communist party. Second, the Prime Minister of Czechoslovakia was a communist; the President and Foreign Minister were not. Finally, the fact that the communists took power in Czechoslovakia by means of an armed coup sent waves of fear through the western world, causing the 'iron curtain' to fall even further. The U.N. had its hands tied, because there were 'free' elections (the candidates were all communist) and there was no proof of Russian involvement. While it cannot be proved that Stalin ordered the overthrow, the signals were clear: Stalin had likely encouraged the rebellion, and it was not coincidental that Russian troops in Austria were moved up to the Czech border. Czechoslovakia was the final east-west bridge, and with the fall of it, the 'iron curtain' was complete.
The final hostile movement of Stalin of importance was the Berlin blockade and airlift. When Russia grew dissatisfied with the economic disparity that had developed in Berlin, it responded by closing all road, rail and canal links between West Berlin and West German. The goal was to force western powers from West Berlin by reducing it to the starvation point.
While the blame for the Cold War cannot be placed on a single man, Stalin's expansionist policy was clearly an ever-present catalyst in the war. Certain Truman was not blameless, but the U.S. was not expanding its empire -- the Soviet Union was. Whether the expansion was for self-preservation, or whether it was merely imperialistic expansion, is relatively irrelevant. What Stalin's actions unarguably did was start a string of chain-reactions within the western powers, and therefore, a good deal of the blame must rest with him.
