Why did President Truman decide to develop the hydrogen bomb?

because we needed an easy way to get past the Japanese without losing to many troops, eventually it was droped on hiroshima, but intresting fact is that america had air raids that droped pamphlets warning civilians to leave, i forgot if it wasent tooken seriously of they were ordered to stay, or what, but then their leader still refused to give up, so the second one was droped on Nagasaki and that ended the war