While in Dresden, he stayed in a slaughterhouse and worked in a malty syrup factory.
Along with the other American prisoners, they managed to escape the massive bombing that killed 135,000 people.
After the bombing, he visited basements in the area. He said it was full of dead people sitting in chairs like they all had a heart attack.
How it influenced Slaughterhouse-Five:
It is obvious that Kurt Vonnegut is anti-war, which is shown in his novel, Slaughterhouse-Five.
Ironically, post-war the protagonist, Billy became a professional optometrist. While Billy helped his patients regain their sight, he lost his "true-sight" because of the war.