What warnings did the US give Japan before they dropped the bomb on Hiroshima?
Military campaigns are secret by their very nature. Bombings over the Japanese mainland were always secret (as were bombings by the Japanese and German militaries) for risk of having bombers shot down. It is likely that some Japanese completely dismissed these leaflets. However, the Japanese government repeatedly lied to its citizens telling them that no bombings could occur (although Japan was being bombed at that point) and that no cities would be destroyed despite the fact that Japan was warned of a new and powerful device that would level cities. This was far more warning than any other people received prior to an attack such as the allied bombing of Dresden, Germany; the unprovoked Japanese bombing of Pearl Harbor in the US; and the month-long massacre of more than 400,000 Chinese residents of Nanking by Japanese soldiers.
Maybe once: The leaflet mentioned below was (by some accounts) dropped on Hiroshima. However of the many versions of the leaflet which mention 33 imperiled Japanese cities, none ever mention Nagasaki, Hiroshima, or Kokura. So perhaps Japanese citizens in Hiroshima et.al. felt that the other cities were going to be bombed but that Hiroshima was not.
On August 1, 1945, five days before the bombing of Hiroshima, the U.S. Army Air Force dropped five million leaflets over Hiroshima, Nagasaki, and 33 other Japanese cities warning that those cities were going to be destroyed within a few days and advising the residents to leave to save their lives. One side of the leaflet had a photo of five U.S. bombers unloading bombs and a list of the targeted cities. The other side had the text.