This Latin phrase means "If you want peace, prepare for war." It comes from the Roman author Publius Flavius Vegetius Renatus around the 4th century BC and conveys the idea held by the Roman Empire that peace is often preserved by the ability to wage war to defend it when the need arises.
The 9mm ammunition cartridge is sometimes referred to as "9mm parabellum" and this reference comes from the original Roman phrase.
