Why did president truman decide to drop the atomic bombs on japan?

While the United States began conventional bombing of Japan as early as 1942, the mission did not begin in earnest until mid-1944. Between April 1944 and August, 1945, an estimated 333,000 Japanese people were killed and 473,000 more wounded in air raids. A single firebombing attack on Tokyo in March 1945 killed more than 80,000 people. Truman later remarked, "Despite their heavy losses at Okinawa and the firebombing of Tokyo, the Japanese refused to surrender. The saturation bombing of Japan took much fiercer tolls and wrought far and away more havoc than the atomic bomb. Far and away. The firebombing of Tokyo was one of the most terrible things that ever happened, and they didn't surrender after that although Tokyo was almost completely destroyed."

In August 1945, it was clear that conventional bombing was not effective.