I looked at that page before writing my comment.
It says that perfidy is a war crime. However, I don’t see anything supporting summary execution.
https://en.m.wikipedia.org/wiki/Summary_execution says the following:
“Francs-tireurs (a term originating in the Franco-Prussian War) are enemy civilians or militia who continue to fight in territory occupied by a warring party and do not wear military uniforms, and may otherwise be known as guerrillas, partisans, insurgents, etc. Though they could be legally jailed or executed by most armies a century ago, the experience of World War II influenced nations occupied by foreign forces to change the law to protect this group.”
