Harry Truman became president under the worst possible circumstances: his predecessor, President Franklin Roosevelt, died during a crucial time in the waning days of the war; he was essentially an emergency replacement for Roosevelt at the conference of allied leaders in Potsdam, Germany, in July 1945, when the fate of post-war Europe was negotiated for the final time with Soviet leader Joseph Stalin; the United States was preparing to test the world's first atomic bomb, the development of which was largely unknown to the new president; and Truman was unskilled in the practice of diplomacy. To complicate matters for Truman, Soviet troops were advancing into areas the resolution of which had yet to be determined, for example, Iran.
All that being said, the fate of Eastern Europe had already largely been decided at the Yalta Conference in February 1945. Roosevelt, then-British Prime Minister Winston Churchill, and Stalin met in that Crimean city prior to Germany's inevitable surrender to discuss the post-war world. The negotiations later that year in Potsdam after the German defeat were intended to formalize the decisions made at Yalta. That Truman was now representing the United States, with Clement Atlee having replaced Churchill as prime minister of Great Britain, is seen by some as having played into Stalin's hands by enabling the ruthless dictator to take advantage of his counterparts' inexperience. Some historians have even suggested that Truman's confrontational temperment hastened the advent of the Cold War, a questionable proposition given what is known of Soviet plans for the post-war continent.
In any event, Truman's response to the division of Europe, in the context of communist activities in Greece and Italy, and the economic devastation wrought by the war was his enunciation in a March 1947 statement to Congress of what became known as the Truman Doctrine. The essence of the Truman Doctrine was for the United States to assume a greater role in European affairs than had previously been the case and to help countries like Greece and Turkey not under the Soviet thumb to defend themselves against communist political activities and insurgencies.
Two events in 1948 in particular provided extra motivation for Truman to act forcefully in Europe. The first was the February 1948 coup in the capital of Czechoslavakia, known as "the Prague coup," in which communists closely allied with the Soviet Union seized total power there, violating Communist Party leader Klement Gottwald's earlier pledge to allow for a democratic transition.
The second, and more significant, event that year was the Soviet imposition of a total blockade around the western half of the city of Berlin -- the half occupied by the United States, Britain and France. The Soviets refused to allow any food or supplies into the beseiged city. Truman's response to the Berlin Blockade was to begin a massive airlift of goods into West Berlin for the year the blockade remained in force.
A major extension of the Truman Doctrine was the Marshall Plan, named for retired general and new Secretary of State George Marshall. The Marshall Plan was designed to inject enormous sums of money into Europe to assist in rebuilding. Stalin's rejection of participation in the plan, and his refusal to allow the East European countries to participate, helped to cement an already enormous chasm between the former wartime allies.