Before Truman became president, he was vice president to Franklin Delano Roosevelt, he had a lack of experience in American politics and so he was in a state of vulnerability when Roosevelt died in 1945, when communism was thriving in much of eastern Europe and the far east.
Truman had represented America during the Potsdam conference of 1945, he was against Stalin who represented the soviet union and Britain, who was represented by Clement Atlee. Like Truman, Atlee had little political experience and they were against an old campaigner, Stalin.
The military situation in eastern Europe ensured that Stalin could get what he wanted to protect the Soviet Union from any future attacks. However, America was developing the atomic bomb, a weapon to be used against the Japanese. Truman told little information about the lethal weapon to Stalin, which had made Stalin grow very suspicious towards the US.
In foreighn policy, Truman was famed for creating the Truman doctrine, this clearly stated that America would defend any state that had a small army that attempted to get rid of the current government. He (Truman) had introduced this doctrine as a result of the Greek Civil War, where communist forces tried to get rid off the Greek monarchy and institute a communist government.
Truman was also responsible for dismissal of General Douglas Mac Arthur, in 1951, during the Korean War for disobeying orders.