Short answers, pulled from the story.
Truman became the 33rd president on the 12th of April, 1945, when Franklin D. Roosevelt died after a massive cerebral hemorrhage. Truman had served as vice president for only 82 days before being sworn in at 7:09 p.m. in the West Wing of the White House by Chief Justice Harlan F. Stone.
Truman authorized the bombings of Hiroshima and Nagasaki after Japan refused the surrender demands of the Potsdam Declaration and with military estimates projecting catastrophic casualties from a land invasion. Studies prepared for Secretary of War Stimson estimated an invasion could cost between 1.7 and 4 million American casualties and up to ten million Japanese fatalities. Truman called it "the hardest decision I ever had to make."
The Truman Committee was a special Senate committee Truman chaired between 1940 and 1944 to investigate waste and corruption in wartime government contracts. The committee reportedly saved as much as fifteen billion dollars and placed Truman on the cover of Time magazine. The Senate's own historical record credited it with erasing Truman's earlier image as a political machine errand-runner.
Truman won with 303 electoral votes to Dewey's 189, despite a spring approval rating of only 36 percent and a Democratic Party fractured three ways by Dixiecrat Strom Thurmond and Progressive Henry Wallace. Truman campaigned by train across the country, with whistle-stop speeches from the presidential car Ferdinand Magellan, and the three major polling organizations all stopped polling before he surged ahead of Dewey.
Truman issued Executive Order 9981 desegregating the United States Armed Forces and Executive Order 9980 prohibiting discrimination in federal agencies, both in 1948. In February 1948 he sent Congress a ten-point civil rights program including anti-lynching legislation and voter rights protections. On the 29th of June, 1947, he became the first president to address the National Association for the Advancement of Colored People.
Historians consistently rank Truman in the first quartile of U.S. presidents, a dramatic reversal from the heavy criticism his administration faced when he left office in January 1953. Critical reassessments of his presidency have improved his standing among both historians and the general public. He is credited with the Marshall Plan, NATO, the Berlin Airlift, the desegregation of the armed forces, and recognition of the State of Israel.