Ruses of war
“All warfare is based on deception. Hence, when we are able to attack, we must seem unable; when using our forces, we must appear inactive; when we are near, we must make the enemy believe we are far away; when far away, we must make him believe we are near.” ― Sun tzu, The Art of War
Kant
Telling the truth, according to Immanuel Kant, is a duty which must be the basis of all duties. If exceptions were admitted or allowed, then the performance of such laws would be rendered uncertain and ineffectual. His prohibition against lying is absolute.
How then would Kant conduct a war, a domain of social phenomena based on deception, if he were a soldier on the battlefield? Will a Kant with a gun risk his life for the sake of the philosophical position against lying?
Or, is there an acceptable form of lying and deception in Kantian version of jus ad bello (ethics for combatants)?
Hague and Geneva Conventions
Kant may or may not have an exception for war but Hague and Geneva Conventions do certainly permit deception and misinformation to an extent.
“Ruses of war and the employment of measures necessary for obtaining information about the enemy and the country are considered permissible." — Hague Convention of 1907
The Geneva Convention, too, does not prohibit misinformation, decoy, and mock operations.
However, there are certain deceptions that are prohibited even in war. For example, one can't kill by misusing the red cross symbol or such other insignia or badges whose misuse will destroy the "good faith," which holds life together universally.