Pretty sure it isn't. I am still 99% sure the GC actually states you have to be identifiable as a combatant 'before, during and after'.So long as you conducted the actual attack in 'uniform', getting within range dressed as a civvy is a legitimate ruse of war.
Edited to add; this is the ICRC definition for it;
Combatants | How does law protect in war? - Online casebookCombatants are required to distinguish themselves from the civilian population while they are engaged in an attack or in a military operation preparatory to an attack.
A combatant who fails to distinguish himself while he is engaged in an attack or in a military operation preparatory to an attack loses his combatant status if he is captured, which means he does not have prisoner ofwar status and can be tried for an act of war.
However, he is to be given protections equivalent in all respects to those provided to prisoners of war under the third Geneva Convention.
Notably, in international armed conflicts governed by Additional Protocol I, a combatant distinguishes himself sufficiently if he carries his arms openly:
- during each military engagement; and
- during such time as he is visible to the adversary while engaged in a military deployment preceding the launching of an attack in which he is to participate.