22Balthazar Ayala, Three Books on the Law of War, 1582 (selection)

Of him who loses, or surrenders to the enemy, a fortress or town which he has been appointed to defend[Vol. 2, 236]

[I]f a man under compulsion of necessity (which, as Livy somewhere says, not even the gods can overcome) and in the utter absence of human aid, surrenders a fortress to the enemy, I hold that he does not deserve punishment; for to do one's best is to do all the law requires, and a vassal only owes fealty to his suzerain within the limits of what is possible, there being no legal obligation to perform the impossible. . . . Hence it is a common doctrine that an ungarrisoned city is quit of the charge of treason should it be surrendered to the enemy, especially if no relief is sent, and that a commander of a fortress is not bound to defend it if the King fails to supply the things that are necessary for its defense. It is on this ground that a vassal is not bound to do his services to his lord, or to recognize him, if he abandons the vassal in time of need. . . .