“He, therefore, who desires peace, should prepare for war. He who aspires to victory, should spare no pains to form his soldiers. And he who hopes for success, should fight on principle, not chance. (Book 3, Foreword)”
Igitur qui desiderat pacem, praeparet bellum; qui uictoriam cupit, milites inbuat diligenter; qui secundos optat euentus, dimicet arte, non casu.
De Re Militari (also Epitoma Rei Militaris), Book III, "Dispositions for Action"
Variant: Si vis pacem para bellum. ("If you want peace, prepare for war.")