When Britain guaranteed Polish neutrality in March 1939, it was a promise that could not be enforced. This convinced Stalin that the west was untrustworthy and full of empty promises and even further so when Britain and France declared war on Germany but did not attack. This "phoney" war was typical of western cowardice.
Another critical factor would be that in June 1940, Stalin would feel he was bearing the brunt of Nazi aggression, the West could have easily defeated Germany while they were preoccupied with Russia (Stalingrad). But instead they made excuses that they were not ready for attack.
These are just some of the incidents in the war, which broke East and Western bonds and caused distrust between these two superpowers. During the second world war these two sides only had an alliance of convenience, once the war was over they had nothing in common except a mutual hate.
In 1947 two declarations were made by the U.S.A which Stalin saw as an attack on communism. The Truman Doctrine and Marshall aid were their names. The Truman Doctrine aimed to support countries, or groups that were being affected by "outside pressures" or "armed minorities". Although Truman did not mention Russia or Communism, it was clear what he meant. Any country feeling threatened by communism would receive aid in the form of money, weapons or advisors. It aimed to stop a western expansion of communism into Europe.
Marshall Aid was an American plan to provide massive financial aid to the economically stagnant countries in Europe after the second world war. This seemed to be not merely an act of kindness, but a way of swaying countries to Eastern Policy, even if it meant America flexing its financial muscle. Although the idea that even Russia could apply for aid seemed feasable, Stalin's spies discovered the government did not really want to give anything to Russia, furthering Stalin's suspision.
These two plans were known as "the stick" and "the carrot", although this was not an act of violence, this was certainly the first act of agression between the two sides, clearly in both plans there was a hidden agenda against Russia.
Lenin had created the comintern in 1917 before the Russian civil war, and this stood for communist international. It was made to encourage an expansion of communism. In 1947 Stalin breathed new life into the organisation and renamed it the cominform (communist information bureau), it was no coincedence this came so swiftly after the Marshall plan and truman doctrine were introduced.
The Comecon (jan 1949) was Stalin's response to the two plans, it primarily aimed to prevent countries being torn from the soviet sphere of influence and to coordinate economic policies of the eastern bloc countries. This brings me onto my next point of why Stalin decided to create "buffer" states.
The Comecon was very unsuccessful, Russia's economy had already been obliterated due to the war, and there was no way Stalin could spare much money for other countries. Nothing like the $14 billion dollars given out by the Americans at least. This meant he had to concentrate on the countries stronger aspects, one of which was their military. Stalin already feeling threatened by the west needed some form of protection, for Russia and Communism as a whole. This is why he created his buffer states in Eastern Europe. The reperations he received in the war such as that of German munitions factories allowed him to reestablish a powerful military. America may have said the reperations were received were too much (50%), but Russia suffered around 27,000,000 military and civilian casualities. Which is more than that of U.S.A, France and Britian all together.
By the end of 1949 there were 9 states in the communist bloc: USSR, Albania, Bulgaria, Czechoslovakia, East Germany, Hungary, Poland, Romania, Yugoslavia. Although Stalin's methods of ensuring communist "influence" were reasonable shady (i.e Marsyk "falling" from a window) they were his only way of truly protecting this eastern form of government. Clearly if these European States were not under soviet influence, communism would have been destroyed. If the tables were turned, the west would have no choice but to do the same.
Although America may have condemned Stalin for spreading eastwards and using dirty tactics, America was far worse. One need look no further than Italy for an example of this. The first general election since world war 2 was due in April 1948. It looked as if the communists may win. American money was channeled, via the newly formed CIA to fund non-communist candidates (this was the CIA's first political operation.) In the USA, America launched a propaganda campaign, 10 million Italian immigrants in the USA sent letters to their Italian relatives encouraging them not to vote communist, explaining how detromental it would be to Europe. It was a hugely successful operation, the communist vote was almost half, and all the communist party were devastated.
Nuclear Tests and Launches (i.e Bikini Atoll and Nagasaki) were another thing that make Stalin's policies seem all the more defensive. It is thought that the launches had an alterior motive which was to alarm Russia, therefore spreading a countries power seems all the more reasonable in light of the power demonstrated by these Nuclear Warheads.