INSIDIA. — The term (from the Latin *insidiae* and *insideo*) refers to deceit prepared secretly against someone and takes on different names according to the various purposes for which it is employed: fraud, deceit, inducement to sin, defamation, injury, murder, temptation; here it is considered insofar as it signifies deceit aimed at misleading the enemy, especially in time of war.
Regarding the morality of such insidiae, Augustine at the time asserted that fighting, even if not openly, but through insidiae, does not violate the virtue of justice; numerous examples from the Old Testament sufficiently prove this doctrine, accepted not only by the Gentiles but also by the chosen people. God Himself commanded Joshua to use insidiae against the inhabitants of the city of Ai (Jos., 8:2, 4, etc.). Therefore, insidiae is not intrinsically dishonest, although the means by which it is carried out may be, for example, when one lies or violates agreements: this is certainly prohibited, as it is contrary to charity when it is not also contrary to justice (Sum. Theol., 2a-2ae, q. 40, art. 3 and q. 71, art. 3, ad 3).
It is possible, however, for the enemy himself to be deceived because something has been kept hidden from him (e.g., war plans or troop movements), or he believes troop movements in one direction to be real when they are simulated. Moreover, in modern international law, whether by custom or express convention, ruses, stratagems, ambushes, feigned retreats, campfires, are commonly used to induce the enemy to believe in the presence of opposing troops encamped. In all these cases, there is no violation of justice or charity, except apparently; for rightly ordered charity begins with oneself. It is, however, illicit to use the enemy’s military uniform, to display the enemy’s war flag on the field, to demand surrender through deceit, or to send military and war objects toward the enemy camp under the insignia of the Red Cross.
Unfortunately, every war weakens the moral principles that should govern relations even between enemies, nor do international laws and conventions suffice to eliminate this effect: for this reason, it is easy to move from simple insidiae to clear and open violations of even natural law.