[AlaskaErik]

If they put on US uniforms they can be considered spies.

Maybe unlawful combatants, but not spies. It's hard to be considered a spy in your own country when you're trying to repel what you consider to be an invader. Even if you are wearing a US uniform.

[Little Bill]

It used to be that dressing in an opposition uniform for the purpose of combat was grounds for summary execution, same for civilians taking up arms, the 1950 Rules of Land Warfare changed that but how, I forget.