My colleague above is right, of course. In my answer, I will endeavor to provide a little more background about the Yalta conference and how decisions made at the conference affected the national security of Eastern European nations.
The Yalta Conference was actually one of three war-time conferences (the other two being the Tehran and Potsdam Conferences) between the three Allied powers of England, the United States, and Russia. The leaders of the three countries walked away from the Yalta Conference with reservations.
Roosevelt found himself in a tight spot; he needed Russian help to fight off imperialistic Japan. Stalin was equally anxious about regaining Soviet national pride after the humiliating territorial losses of the Russo-Japanese war. He promised Roosevelt the help he requested on the condition that Soviet influence in Eastern Europe would be recognized by the Allied powers. After all, the Soviets were already in Poland; Stalin wasn't about to give up what he believed was a prime military advantage. Securing the national security and political relevance of Russia was his main priority.
"For the Russian people, the question of Poland is not only a question of honor but also a question of security. Throughout history, Poland has been the corridor through which the enemy has passed into Russia. Poland is a question of life and death for Russia."
On the other hand, Churchill maintained that free elections in Eastern Europe would ease English concerns about Russian military aggression and imperialistic ambitions. Both Churchill and Roosevelt believed that the Polish government-in-exile in London was the legitimate choice for a free Poland. However, Stalin's Red Army had already set up a pro-communist Polish Provisional Government.
With Russia having removed most of the Nazi forces from Eastern Europe, both Roosevelt and Churchill faced checkmate in post-war negotiations. In the end, the prospect of free elections in Eastern Europe was sacrificed for the sake of stability after WWII. With Russia defending its national interests throughout Eastern Europe, a new Cold War between the United States and Russia would last almost 40 years as a result of this uneasy truce.