According to president Truman, for what reason was the atomic bomb used on Japan?

To end the war once and for all. An invasion of Japan would’ve led to millions more killed and Truman felt the a bomb was the best option to avert that

Germans were already defeated, so this only left Japan. Truman knew that if we kept fighting that there will be lots and lots of casualties. So Truman had no choice but to make the difficult decision on dropping the bomb on Nagasaki and Hiroshima. So to sum it up the reason was to prevent any more casualties and to end the war.