“Prepare for war so you don’t have to fight one” is a paraphrase of the sage wisdom of the ages.
The original Latin of the expression “if you want peace prepare for war” comes from “Epitoma Rei Militaris,” by Vegetius. The Latin is: “Igitur qui desiderat pacem, praeparet bellum.”
http://ancienthistory.about.com/od/warfareconflictarmor/f/PrepareforWar.htm