Saw your sig, just FYI:
It is a tweaked Latin phrase, the origin exactly is "Igitur qui desiderat pacem, praeparet bellum" (All you that want peace prepare for war) by Flavio Vegecio Renato from his book "Comendium of Military Techniques" pubished in 390, same meaning, but different layout (not so imperatively, more as hypothesis). It often gets attributed to Julius Cesar, but that is wrong.
Rattler