I've been reading (well, mostly skimming) the stories in The Best of Lester del Rey, and just found this line:
"War was a dangerous form of political insanity, outlawed at the conference of 1998."
--from "Vengeance Is Mine," 1964
. . . Added later, after finishing reading the story: It turns out that this was actually a quite deliberate choice on the author's part, and not at all the old-school-skiffy techno-rationalist optimism that I had thought it was; the character who says it is a robot from whom most of human history has been concealed. The story turns out to be a whole lot darker than it initially looked, and it started out pretty dark. Interesting story.
I was mildly amused at the meta-joke a little later in the story, as the robot reads through a large library of books and starts to understand (and misunderstand) the concept of fiction: "In the end, he evolved a simple classification. If a book made him think hard and forced him to strain to follow it, it was fact; if it made him read faster and think less as he went through it, it was fiction."