Whether It Is Lawful to Lay Ambushes in War?

Objection 1: It would seem that it is unlawful to lay ambushes in
war. For it is written (Deut. 16:20): "Thou shalt follow justly after
that which is just." But ambushes, since they are a kind of
deception, seem to pertain to injustice. Therefore it is unlawful to
lay ambushes even in a just war.

Obj. 2: Further, ambushes and deception seem to be opposed to
faithfulness even as lies are. But since we are bound to keep faith
with all men, it is wrong to lie to anyone, as Augustine states
(Contra Mend. xv). Therefore, as one is bound to keep faith with
one's enemy, as Augustine states (Ep. ad Bonif. clxxxix), it seems
that it is unlawful to lay ambushes for one's enemies.

Obj. 3: Further, it is written (Matt. 7:12): "Whatsoever you would
that men should do to you, do you also to them": and we ought to
observe this in all our dealings with our neighbor. Now our enemy is
our neighbor. Therefore, since no man wishes ambushes or deceptions
to be prepared for himself, it seems that no one ought to carry on
war by laying ambushes.

_On the contrary,_ Augustine says (QQ. in Hept. qu. x super Jos):
"Provided the war be just, it is no concern of justice whether it be
carried on openly or by ambushes": and he proves this by the
authority of the Lord, Who commanded Joshua to lay ambushes for the
city of Hai (Joshua 8:2).

_I answer that,_ The object of laying ambushes is in order to deceive
the enemy. Now a man may be deceived by another's word or deed in two
ways. First, through being told something false, or through the
breaking of a promise, and this is always unlawful. No one ought to
deceive the enemy in this way, for there are certain "rights of war
and covenants, which ought to be observed even among enemies," as
Ambrose states (De Officiis i).

Secondly, a man may be deceived by what we say or do, because we do
not declare our purpose or meaning to him. Now we are not always
bound to do this, since even in the Sacred Doctrine many things have
to be concealed, especially from unbelievers, lest they deride it,
according to Matt. 7:6: "Give not that which is holy, to dogs."
Wherefore much more ought the plan of campaign to be hidden from the
enemy. For this reason among other things that a soldier has to learn
is the art of concealing his purpose lest it come to the enemy's
knowledge, as stated in the Book on _Strategy_ [*Stratagematum i, 1]
by Frontinus. Such like conc