The Soviets 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 hostility 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 wary because he was not informed of U.S. nuclear capabilities, therefore a lack of trust between the major powers was existent.
The Russians were also made an enemy of by Kennan's Long Telegram coupled with Churchill' Iron Curtain speech. The telegram stated that the Russians are 'aggressive expansionists.' Not only does this follow on from my previous point that the USA vilified the Soviets but also the Russians expanded gradually, this disproves a large chunk of the telegram as it cannot be aggressive expansionism if the Russians were expanding gradually and through elections often. However there is evidence that shows although the Soviet expansion was gradual, it was not always done sincerely like agreed at the Yalta and Potsdam conferences it was often the case in the satellite states, that the elections that had been agreed to take place to determine the countries futures, were often rigged and would therefore put the communists in power, so the agreements at the conferences were not obeyed.
However, in relation to the telegram and the content of what Kennan said, there is evidence behind it that also helps disprove the statement. Communism is a major factor in itself due to a key element of it being that it should be international, and spread throughout the world. The belief of the Soviet Union was that communism and revolution should be spread across the world so therefore this is the basis behind Kennan's opinion. Although the ideology states that communism should be international Stalin had already gone against this himself, through socialism in one country policy to make the Soviet Union strong, so whilst he strengthened the USSR, Stalin went against communism beliefs as a whole.
Also, another factor which opposes the justification of expanding into Eastern Europe for ‘security’ reasons, is the sheer amount of eastern European countries that the Soviet Union occupied. Albania, Bulgaria, East Germany, Finland, Romania, Poland, Czechoslovakia and Hungary were all a part of Soviet expansion. Even if Stalin wanted a ‘buffer’ zone, Britain and the USA were suspicious upon Stalin due to the amount of countries that they occupied. A British cartoon in 1949 illustrated this, as it presented Stalin overlooking a map of Eastern Europe, showing how he reigned upon the ‘vulnerability’ of these countries. An American cartoon at around the same time also illustrated this with more and more buffers around buffers, to show how Stalin was manipulating the idea of a buffer zone for security reasons. Britain and the USA, ultimately though that Stalin was exploiting the weaknesses of vulnerable Eastern European countries, aswell as exaggerating the use of the Soviet Union’s buffer zone. To the western world, this looked as if it were the beginning of serious Soviet aggressions, therefore naturally the Western powers were feeling defensive, and wary of Stalin’s plots.
In conclusion, I believe the statement that the Soviet Union only expanded into Eastern Europe for security reasons is valid. Although the western powers believed that Stalin was over utilising a ‘buffer zone’ as an excuse, to continue to dominate Eastern Europe, in my opinion, Stalin was seriously concerned about another Western invasion, and he felt more secure with other Soviet occupied Eastern European countries surrounding the USSR. Although Stalin may have expanded too much, and consequently made the western countries feel defensive, I believe that Stalin was mainly concerned about security and paranoia may have led to the worries of Britain and the USA in particular.