Final answer:
President Truman was motivated to use the atomic bomb to end WWII promptly and save American lives, justify the costs of the Manhattan Project, and demonstrate power to the Soviet Union, despite some military leaders who believed it unnecessary.
Step-by-step explanation:
President Truman felt strongly about using the new technology, the atomic bomb, primarily due to the desire to save American lives that could potentially be lost in a prolonged war with Japan. Another consideration for Truman was the enormous investment in the Manhattan Project; the successful Trinity Test had demonstrated the bomb's immense power, and using it could justify the project's cost. Furthermore, there was an underlying strategic motive to demonstrate to the Soviet Union the technological capabilities of the United States at the end of World War II.
Truman, who had only become president shortly before the bomb's completion, was influenced by his advisors who held a strong belief in both the justice of the U.S. cause and technological solutions for ending the war. There were differing opinions, however, as some military leaders felt Japan was nearing surrender due to the effective naval blockade and the fire-bombing of Japanese cities.
In balance, Truman favored using the bomb against military rather than civilian targets. His decision was shaped by the potential to end the war swiftly with fewer casualties than a full-scale invasion would incur.