Why did president harry truman decide to hit the japanese cities of hiroshima and nagasaki with two atomic bombs in 1945?

He believed that if he attacked first he would save thousands of American lives if he scared the Japanese into surrendering

He wanted the Japanese to surrender unconditionally. He dropped 2 because he thought that if he only dropped 1 bomb psychologically the Japanese would not surrender. So 2 would make them think that we have more and will keep dropping them.