How did Russian Expansion happen?

Poland

In 1945 Stalin under pressure from the West set up a provisional government which included members of the exiled government. Mikolajcyk was the leader of the government who had set up in London during the war and he returned to be part of this new provisional government. Stalin announced that Poland would have an election. Behind the scenes Stalin knew that the election would not be free as it was likely that he would lose control of the government. When Mikolajcyk found out the elections would not be free he resigned and stopped his party, the Polish Peasants Party, from joining these elections. He hoped the USA would realise that these elections were one sided and intervene to stop them.

Unfortunately for Mikolajcyk, on the USA side they too were working behind the scenes. Truman had accepted that Poland was in the Soviet sphere of influence and that there would be little he could do to prevent Russia taking control. Both Britain and USA declined Mikolajcyk's invitation to send delegates to monitor the election.

As a result through the use of terror and falsified election results the Soviet Communist party on a massive majority. The leader of the new Communist Polish government was not completely happy. He believed that while he was dependant on Soviet assistance that Poland should be independent from Soviet Russia. He was disappointed with the development of Cominform and only under a large amount of pressure did he accept that he would have to accept a Mosow model of Socialism. A year later Stalin replaced him with a leader who could be more easily controlled.

Romania
There was very little dispute about who should have influence in Romania after the war. the Western powers agredd that it was vital security zone for Russia. There was no strong opposition like there had been in Poland and so the Soviets were able to establish themselves much quicker here than elsewhere.
In March 1946, following an election the communists gained a majority of 80%.