Specifically, Vonnegut was present at and survived the bombing of Dresden, which became part of (the main?) the inspiration for Slaughterhouse-Five: https://en.wikipedia.org/wiki/Kurt_Vonnegut#World_War_II

The book makes a lot more sense once you understand Vonnegut's history with the bombing.

Sadly, Vonnegut quotes David Irving's "The Destruction of Dresden" as a source for the number of dead (extremely inflated over other sources). Sadly because although Vonnegut didn't know it at the time, Irving was a Holocaust denier who wanted to rehabilitate the reputation of the Nazis by showing them to be no worse than the Allies.