About an hour before the bombing, Japanese radars detected the approach of American aircraft. An alert had apparently been sounded and radio broadcasting stopped in a number of cities. The planes approached at a high altitude and at around 8 am, a radar operator realised that the number of aircraft was small.
Assuming this couldn't be much of a threat, the alert was removed. The Japanese thought it was just a reconnaissance mission.
They could not have been further from the truth.
Nagasaki was destroyed by another atomic bomb dropped by the US military three days later, on August 9. In all, approximately 120,000 people were killed outright. The attack had, in every single detail, been carried out exactly as planned.
Also See: Auschwitz: The evil men did
External link: The Sun Sentinel: Will we take opportunity to reflect?