"The one who wishes for peace, should prepare themselves for war" is a quote traceable all the way back to Vegetius (Publius Flavius Vegetius Renatus) in the late fourth century.
He had it as Qui desiderat pacem, praeparet bellum. It was later shortened into "Si vis pacem, para bellum" (If you want peace, prepare for war), the last two words of which then got mashed together to give another name to the Luger handgun.