Why did America drop an atomic bomb on Japan?

Why did the Japanese , torture and kill 300,000 Chinese in 1937 in what was called the of Nanking? Answer; to send a message to the Chinese government that they were capable of going block by block and wipe out and entire city and its' citizenry in one month and there was nothing the Chinese government could do to stop them, and as a warning that more devastation would ensue if they tried. This is basically what we did to stop the Japanese empire from continuing the war. The weapon was new, incredible, decisive, and utterly destructive. We had to drop two because they didn't think we had more than one. We were ready to drop another on Kokura on August 14th, 1945 but the Japs surrendered before that. Also the Russians declared war on Japan on August 8th, 1945 and invaded Manchucko. The Japanese army was demoralized and ran off the fields of combat. The Japanese government felt they could occupy Manchuko if their homeland was destroyed and move into that territory. This mode of thought obviously changed after Aug 8th and after the second A bomb was dropped the government was left with no other option but to surrender unconditionally.