Stalin's chief security concern in the immediate postwar era was to secure the western border of the Soviet Union. In the first half of the twentieth century, the Russian people had been involved in two unspeakably destructive conflicts that originated in Europe. Both of these conflicts led to the invasion of the Russian homeland, and a central concern of the Soviet Union was to establish friendly (i.e. communist) governments in the nations of Eastern Europe, especially Poland. Even as the war was coming to an end, Stalin maneuvered to install a communist goverment in Poland, which the new US president Harry Truman understandably viewed as a breach of agreements made to then president Roosevelt at the Yalta Conference.
The Soviet Union facilitated the rise of similar governments in other eastern European nations over the next three years, with a brutal coup in Czechoslovakia finalizing control over what would become known as the Eastern Bloc. For their part, the United States and Great Britain interpreted Stalin's actions, particularly in Czechoslovakia, as expansionist. The policy of the United States, beginning in Greece and Turkey, and continuing in West Berlin, became to "contain" Soviet expansion through diplomatic maneuvering, economic aid, and, if necessary, the threat of military intervention.
Other security concerns included the future of Germany, which Stalin hoped to keep militarily weak, Soviet desires to exercise influence in postwar Japan, the outcome of the ongoing civil war in China, and particularly the development of atomic weapons by the United States.