Truman became president after the death of President Franklin Delano Roosevelt. He immediately met with Roosevelts cabinet and made it clear he’d be making the decisions from this point on. He attended the Potsdam conference where he revealed the existence of a new and terrible weapon that he would be using on the Japanese to Stalin, but was criticized for not taking a tougher stance on the Soviets establishing their block.
After the dropping of the Atomic Bomb and surrender of the Japanese, Truman supported the establishment of the UN and reaffirmed commitments to international cooperation. He supported the Marshal Plan for rebuilding Europe and hoped that by getting the war-ravaged economies back on their feet he could stave off communisms spread. But when communist nations began reneging on their commitments to democracy, he took a hard line and established the Truman Doctrine. It set a precedent were the U.S. would intervene to prevent the spread of communism into democratic parts of the world. The U.S. supported Iran, Greece and Turkey with money and material in order to stave off communism in those countries.
Although this may seem harsh, Truman did his best to avoid unnecessary conflict wherever he could. When the USSR cut off access to West Berlin, Truman ignored recommendations by his cabinet to send in armed convoys and instead instated the Berlin Airlift, which dropped food and other supplies by airplane for over a year. Truman also sent the U.S. 7th fleet into the Taiwan Strait to prevent further fighting after the forced of Mao Zedong forced Chang Kai-shek out of mainland China. He could have escalated the conflict, but decided against further action.
When the Korean War broke out, he sent U.S. troops, backed by a UN peacekeeping force, to help support the poorly-equipped South Korean army. He made many unpopular but sound decisions, such as firing MacArthur and his then Secretary of Defense Louis A. Johnson. In the end he fought off calls for his impeachment and managed to negotiate an armistice.