From its subtitle, "The Children's Crusade", Vonnegut's novel clearly characterizes war as a terrible thing in general and certain aspects of conformity and patriotism as being terrible in particular.
For Vonnegut, war is not an enterprise of glory and heroism, but an uncontrolled catastrophe for all involved, and anyone who seeks glory and heroism in war is deluded. (eNotes)
Billy Pilgrim's experiences in the war are a mixture of the negative, the violent, and the absurd. Vonnegut makes his opinions on war clear from the outset in the novel, using a critical term in the introductory section of the novel:
In a sentence directed to his publisher, Vonnegut said of the novel, ‘‘It is so short and jumbled and jangled, Sam, because there is nothing intelligent to say about a massacre."
"Massacre" here suggests Vonnegut's attitude toward war, which is clearly condemnatory throughout the novel in its depictions of cruelty, negligence and stupidity on the part of all parties involved in running the way. Billy's story and many important anecdotes (i.e., Edgar Derby) paint a severely negative picture of war and its practical realities.