All things that real militaries have done and continue to do. It was in TOS. Meyer's interpretation of Starfleet is perfectly consistent with how it was depicted in TOS. Roddenberry only claimed otherwise because he was pissed that Paramount removed him from authority over the movies after TMP. UN Peacekeepers are military personnel, on detached duty from their home nation's armed forces. Even in today's militaries, not everyone is a combatant. And even in Starfleet, those who are "essentially scientists trained as explorers/astronauts" are still expected to be armed when they are in the field, and if necessary are called upon to fight and kill while in the field.