Before the Atomic bombs were dropped the Americans were dropping explosive and incendiary fire bombs. They dropped them in such massive numbers that entire cities were destroyed resulting in the deaths of hundreds of thousands of innocent civilians. To the Japanese military it didn't make a difference to them at the moment if their cities were being destroyed by regular bombs or atomic bombs. The result was still the same, a completely destroyed city. It wasn't until they found out how effective the weapon was that they decided to surrender, even then many hard liners in the Japanese military wanted to continue the war.