When I was in Germany I saw what I knew was an ambulance being used for ammunition
resupply. The medical vehicles all had different roofs that made them impossible to
misidentify. Being an MP, I asked the observer if that wasn't the biggest violation of the rules of war he had ever seen. He replied that as long as the red cross is hidden from view, the vehicle was a combatant. I looked it up and it's the insignia, not the type of vehicle that
conveys protection. During field problems the OPFOR used to drive up to our aid stations
in a marked red cross vehicle, open the rear doors, and open fire. I got my ass in a sling for arguing that using a red cross vehicle for combat action voided the protection. I won my arguement, but I got in a little trouble for lighting up an ambulance later in the exercise.