It was the end of European War and the Allies focused their efforts on Japan. Although being badly hurt by the bombing and blockade, Japan still wasn't going to give up and fought. The main point of this event was the Emperor, which the Japanese believed to be God.
The bomb hit Hiroshima at 8:15am (Japanese Standard Time)
A proclamation was then issued to Japan, called the Potsdam Proclamation, which demanded Japan to surrender. This proclamation made no mention of the Emperor's status. Japan had rejected the proclamation. Soon the U.S. had dropped their first atomic bomb on Hiroshima and Nagasaki. Then Russia declared war against Japan, which they refused to surrender. The Japanese advocates feared this was a threat toward the Emperor. The Emperor finally called for peace. When the Emperor called for surrender, the entire Japanese cabinet agreed to surrender.
In July 1945 Japan seeked Russia's help to end the war. This message was intercepted by the U.S. This led to the devistation at Hiroshima. Even after the Atomic Bomb, japan was allowed to retain their Emperor anyways.
source - Doug Loug