Cicero in response to Hamdan v. Rumsfeld:
IV. atqui, si tempus est ullum iure hominis necandi, quae multa sunt, certe illud est non modo iustum verum etiam necessarium, cum vi vis inlata defenditur… insidiatori vero et latroni quae potest inferri iniusta nex?
quid comitatus nostri, quid gladii volunt? quos habere certe non liceret, si uti illis nullo pacto liceret. est igitur haec, iudices, non scripta, sed nata lex, quam non didicimus, accepimus, legimus, verum ex natura ipsa adripuimus, hausimus, expressimus, ad quam non docti sed facti, non instituti sed imbuti sumus, ut, si vita nostra in aliquas insidias, si in vim et in tela aut latronum aut inimicorum incidisset, omnis honesta ratio esset expediendae salutis. silent enim leges inter arma nec se exspectari iubent, cum ei qui exspectare velit ante iniusta poena luenda sit quam iusta repetenda.
etsi persapienter et quodam modo tacite dat ipsa lex potestatem defendendi, quae non hominem occidi, sed esse cum telo hominis occidendi causa vetat, ut, cum causa, non telum quaereretur, qui sui defendendi causa telo esset usus, non hominis occidendi causa habuisse telum iudicaretur. quapropter hoc maneat in causa, iudices; non enim dubito quin probaturus sim vobis defensionem meam, si id memineritis quod oblivisci non potestis insidiatorem interfici iure posse.
IV. But if there is any occasion on which it is proper to slay a man, and there are many, surely that occasion is not only just, but even necessary, when violence is offered, and must be repelled by violence… And what death can be unjust when inflicted on a secret plotter and outlaw?
Why do we have an army, why do we own swords? Surely it would not be justifiable for us to have them at all, if it were never justifiable to use them. There is, therefore, a law, O judges, not written, but born with us, which we have not learnt, nor received by tradition, nor read, but which we have taken in and imbibed from Nature herself; a law which we were never taught, but for which we were made, which we were never trained in, but which is ingrained in ourselves: namely, that if our life is in danger from plots, or from open violence, or from the weapons of brigands or enemies, every means of securing our safety is honorable. For the laws are silent in the midst of the clash of arms, and do not expect themselves to be waited upon, when he who waited would be obliged to bear an unjust injury rather than exact a just punishment.
The law very wisely, and tacitly, gives a man the right to defend himself, and it does not merely prohibit homicide, but forbids anyone carrying a weapon for the purpose of murder. It is the intended purpose, not the carrying of the weapon, which constitutes the offense. The man who used a weapon to defend himself would not be deemed to have armed himself with the intention of committing murder. Let this principle then be remembered by you in this trial, O judges; for I do not doubt that I shall make good my defense before you, if you only remember, that which it is impossible to forget: that a plotter against oneself may be lawfully slain.
-Marcus Tullius Cicero, PRO T. ANNIO MILONE ORATIO, [In Defense of Titus Annius Milo], X:IV.