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I suppose the dilemma which makes the UN toothless is that, while many join the armed forces, fully prepared to die for their country, few would be willing to die on the UN's behalf. You could, potentially be shoved into any troublespot, on the other side of the world and become the meat in the sandwich because you're denying TWO armies of an objective.
The UN is just about capable of turning out a force large enough to suppress two conflicting groups of bandits (they suffered losses in Somalia, you'll recall) but they wouldn't be able to turn out a force capable of holding two seriously equipped countries apart.
Worse still, the UN draws units from multiple nations for any given PK action so some permutations of conflict could result in some of the troops in pale blue facing up against those of a former peacetime ally.
As we have all been pained to notice, the USA does not play 'peacekeeper'. They steam in, bring about regime change, then create a power vacuum when they pull out.
(Even the troops they trained up, when it came to the crunch basically ditched their tanks, armoured cars, even their uniform and then ran away because they were only in it to draw a salary - not to get killed, fighting IS).
Who saw that coming?