A better translation (subjectivly better, it sounds better and is more accurate word for word) is:
"If you wish for
peace, prepare for
war".
It more or less implies that war is the natural state of
humanity and peace can only be imposed by force. A neat
metaphor for modern american
foreign policy.
Another tidbit: If you have ever heard of pistol ammunition called
9mm parabellum, it means just that: 9mm prepare for war.