For laws are silent when arms are raised.

[Silent enim leges inter arma.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher

Pro Milone, ch. 4, sec. 11 [tr. Yonge (1891)]
    (Source)

In context, Cicero is asserting that self-defense is a valid defense for killing, even though that principle was not written into Roman law. It has been extended in legal terms to times of war being exempt from normal laws regarding killing.

Alt. trans.:

• "For laws are silent among arms."
• "In a time of war, the law falls silent."
• "Laws are silent in time of war."
• "The laws are silent in warfare."
• "For among arms, the laws fall mute."
• "The power of law is suspended during war."

Original Latin.