If I remember correctly, most of the time the UN charters a nations military to accomplish some mission.

But usually you get a bunch of UN Politicians who never served in the military, but are "so much smarter than those jar-heads" to write up the rules of engagement. So, depending on who sends the troops with the blue hats, either there are professional soldiers who gets shit ROE and really don't want to be there because they usually have to sit around with their thumbs up their ass while people shoot at them. Or you get a less professional military (or has officers who are protected by their country) who don't give a shit about ROE, and will either accomplish the mission or just fuck over the locals.