Short answers, pulled from the story.
Harry S. Truman's presidency began on the 12th of April 1945, when he succeeded Franklin D. Roosevelt upon Roosevelt's death, and ended on the 20th of January 1953. He served one partial term and one full term, winning the 1948 presidential election by narrowly defeating Republican Thomas E. Dewey and Dixiecrat Strom Thurmond.
Truman approved the atomic bombings of Hiroshima on the 6th of August 1945, and Nagasaki three days later to end the Pacific War quickly and minimize casualties. After leaving office, Truman stated the decision was made to save 125,000 American and 125,000 Japanese lives that would have been lost in an invasion of the Japanese mainland.
The Truman Doctrine was announced in a March 1947 speech to a joint session of Congress, calling for the United States to support free peoples resisting subjugation by outside pressures. Truman requested an unprecedented $400 million aid package for Greece and Turkey to prevent Soviet-aligned governments from taking power in those countries.
Truman dismissed MacArthur in April 1951 after House Minority Leader Joseph Martin made public a letter from MacArthur criticizing Truman's handling of the Korean War and calling for escalation against China. Truman believed MacArthur had overstepped his authority by trying to make foreign and military policy, threatening civilian control of the military.
Truman's approval rating stood at 22 percent in Gallup polls in February 1952, which was the lowest recorded for a sitting American president until George W. Bush reached a similar mark in 2008. His unpopularity was driven primarily by the Korean War stalemate and his dismissal of General MacArthur.
The National Security Act of 1947, signed by Truman, created the U.S. Air Force, the Central Intelligence Agency, and the National Security Council, and merged the Departments of War and Navy into the National Military Establishment, later renamed the Department of Defense. Truman also secretly created the National Security Agency in 1952 and helped establish NATO in 1949.