What is the generally accepted penalty, internationally, for soldiers that disguise themselves as civilians and then enter combat under such a disguise?
They lose the protections of the Geneva Conventions. This is what the U.S. used to justify the Gitmo detention, saying that al-Qaeda members don’t wear uniforms (“fixed distinctive sign”) or obey the laws of war. Rumsfeld labeled them “unlawful combatants,” and said the rules of the Geneva Convention did not apply. They can be charged and put on trail
(Here it gets hazy Europe & Canada say there should be an impartial judge who decides whether or not they are “unlawful” that the U.S. can’t just proclaim them so – Rumsfeld/the Administration disagrees)
BTW Even if they are found to be “unlawful combatants,” they still have rights under international law – not to be tortured, to humane treatment and to a fair trial if charged with a crime.