I think a crucial part of it is also that you, as a simple soldier on the ground, don’t really have a good way of figuring out the big picture.

If your sergeant tells you to “prevent anyone from entering or leaving the parliament building”, you’re very likely to assume that something bad is happening and that the army has been called in to secure the building. You basically have to trust your commanders to see the bigger picture, so that when they tell you that “the guys over there are the baddies”, you can engage them without walking over to check for yourself. If those guys are in police uniforms, that probably means the baddies got a hold of uniforms to try to sneak past you.