A ruse of war is an action taken by a belligerent in warfare to fool the enemy in order to gain intelligence or a military advantage against an enemy.

Good faith

Good faith in dealing with an enemy must be observed as a rule of conduct; but this does not prevent measures such as using spies and secret agents, encouraging defection or insurrection among the enemy civilian population, corrupting enemy civilians or soldiers by bribes, or inducing the enemy’s soldiers to desert, surrender, or rebel. In general, a belligerent may resort to those measures for mystifying or misleading the enemy against which the enemy ought to take measures to protect itself.

Legitimate ruses

Legitimate ruses include:

• surprises, ambushes, feigning attacks, retreats, or flights;
• simulating quiet and inactivity (to lull the enemy into complacency);
• use of small forces to simulate large units (for example, it is a perfectly proper ruse to induce an enemy unit to surrender on the ground that it is surrounded and thereby induce such surrender with a small force);
• transmitting false or misleading radio or telephone messages;
• deception of the enemy by bogus orders purporting to have been issued by the enemy commander;
• making use of the enemy’s signals and passwords;
• pretending to communicate with troops or reinforcements which have no existence;
• deceptive supply movements (which might make the enemy think you are preparing an action you're not);
• deliberate planting of false information;
• use of spies and secret agents;
• moving landmarks (to confuse an enemy operating in unfamiliar territory);
• putting up dummy guns and vehicles or laying dummy mines;
• erection of dummy installations and airfields (to intimidate or encourage useless attack);
• removing unit identifications (but not those that identify the belligerent while in combat) from uniforms; and
• psychological warfare activities.

No treachery or perfidy

Ruses of war are legitimate so long as they do not involve treachery or perfidy on the part of the belligerent resorting to them. They are, however, forbidden if they contravene any generally accepted rule.

Prohibited ruses

The line of demarcation between legitimate ruses and forbidden acts of perfidy is sometimes indistinct. In general, it would be an improper practice to secure an advantage over the enemy by deliberate lying or misleading conduct which involves a breach of faith, or when there is a moral obligation to speak the truth. For example, it is improper to pretend to surrender to secure an advantage over the opposing belligerent.

To broadcast to the enemy that an armistice had been agreed upon when such is not the case would be treacherous. Abuse of the protections afforded to medical personnel (by disguising combat soldiers as medics, or by putting a red cross on a combat vehicle) is also considered unacceptable.

It is especially forbidden to make improper use of a flag of truce, of the national flag, or of the military insignia and military uniform of the enemy, as well as the distinctive badges of the Geneva Convention in a ruse of war. [Annex to Hague Convention No. IV, 18 October 1907, embodying the Regulations Respecting Customs of War on Land., art. 23, par. (f).]