Surprisingly, Tokyo first found out about what caused the disaster a whole 16 hours after the attack. They received their information from a White House public announcement in Washington!
The first city in history subjected to nuclear warfare, Hiroshima was founded in 1589, on the coast of the Seto Inland sea. It soon became a major urban centre. During the First Sino-Japanese war, it emerged as a major supply base for the Japanese military and continued to be one during World War II.
It was the primary target of the first US nuclear attack mission and a major factor leading to the surrender of the Japanese government six days after the attacks.
In fact, one of the primary reasons given for the use of the bomb was it would force Japan to surrender unconditionally. The country went on to present its formal document of surrender to the Allied powers on August 15.
Also See: The war that changed the world
External link: The San Francisco Examiner: Local physicist recalls bombing of Hiroshima