After the second world war in 1945 tensions between the Soviet Union and US resumed and escalated in 1945-1947, mainly because of Soviet actions.
The first soviet actions were introduced during the conference in Yalta in february 1945. The big three of the second world war (Roosvelt, Churchill and Stalin) met to discuss following steps, but there were seen first disagreements about soviet actions. One of the soviet demands was moving boarders of poland westwards towards germany which was not negotiable because whole poland was occupied by ussr troops "the red army". Stalin defended his demand by talking about safety of Ussr and claiming that poland is not only question of honor but also question of security and that poland has been throughout the history corridor through which the enemy has passed in to russia. Although Stalin promised free elections there was installed new communist government and it became apparent that Stalin didn't plan to keep his promise.
The three had also decided that all the Europen countries will have original goverment, that democracies will be estabilished and they will hold free elections, and here they also agreed that eastern europe will become soviet's sphere of influence which has proven extremely bad idea because till next conference in potsdam in july 1945 Stalin has had in all eastern european countries installed new communist or pro-communist goverments and they certainly didn't held free elections. Stalin again defended his actions with his desire to make USSR safe. This way he created satellite countries around the USSR showing his panic and hate of capitalism, which however has already proven much more convenient because ussr with communism was the one with money deficit and poverty. But Stalin sticking to his ideology kept on taking over more countries either by insatalling new goverments or by force, many of which has not even had boarders with Russia, like Albania or Yugoslavia. Even more suspicious was his attempt to insatall communist goverment to Greece which was far away from USSR. It may be true that Stalin was only afraid of capitalism and cared only about safety of Russia but the fact is that when all the satellite countries became communist they became part of Soviet Union which meant that other countries next to them in west could be again in possible danger of soviet invasion because of Stalin's fear of capitalism. Stalin might just want to defend USSR but his defending was mostly in terms of expanding, which could possibly continue and communism would spread to the whole of Europe so that Stalin could hopefully feel safe at last.