Final answer:
Harry S. Truman became the President after Roosevelt's death in 1945 and ordered the dropping of atomic bombs on Hiroshima and Nagasaki, leading to the end of World War II.
Step-by-step explanation:
Upon the death of Franklin Delano Roosevelt in 1945, Harry S. Truman took the oath of office and became the 33rd President of the United States. Truman, who had not been privy to the Manhattan Project prior to becoming president, faced the formidable task of concluding World War II. After being informed about the atomic bomb and witnessing its destructive capacity during the Trinity Test, Truman made the monumental decision to use the weapon against Japan. The first atomic bomb named 'Little Boy' was dropped on Hiroshima on August 6th, and the second bomb, 'Fat Man', on Nagasaki on August 9th. These actions ultimately led to Japan agreeing to an unconditional surrender, marking the end of the war.