Dresden is a city in Germany.
During World War II, Dresden was heavily bombed by the United States of America and the United Kingdom. During the raids, the German inhabitants would take shelter in bunkers. On several occasions, the intense heat produced by the bombing would heat the shelters to extreme temperatures. When the bunkers were opened, the oxygen entering the shelters caused the bodies of the people inside to burst into flames.
Keiko O'Brien taught her class, including Jake Sisko, about the bombing of Dresden. Prior to this, Jake had been unable to remember whether Dresden had been bombed during World War II or World War III. (DS9 novel: Fallen Heroes)