As for who makes a decision to go into a country ... well, if you're squads of people from Peru and Japan etc. and there's escalating sectarian violence in Iraq, then you are welcome to go there. It's all decentralized. It's sort of like "actors" in Erlang, each running their own program and enforcing on others.
The only thing that's sort of more centralized would be the due process for those peacekeepers or groups of peacekeepers who violate rule #1. Besides this, peacekeepers would try to screen out peacekeepers who travel to an area where they are found to have a significant stake in the outcome.
Of course, this force would not need any weapons of mass destruction, nuclear weapons, etc. All those could be gradually destroyed by the various nations. They would, however, have crowd control non-lethal weaponry.
My point is that from a utilitarian point of view, it's even better to kill all the belligerents early (after warning them) than let the thing escalate into, say, the Russian Revolution where 20 million people died in fighting. Just the chilling effect alone that "violence only backfires" (analogue of "crime doesn't pay") will deter many from joining the escalation. As opposed to now where the US "world police" merely takes one side and arms them to demolish the other.
The fact that, currently, the powerful countries of the world would not go along with it does not make the idea stupid / horrible.
For what it's worth, something like the UN even existing and many nations having signed arms control treaties was unthinkable prior to WW2.
The only open question I have - who decides upon engaging? The UN Security Council which should deal with issues is effectively gridlocked as the Russians and Chinese veto everything.
The UN General Assembly isn't that much trustworthy either, given how many countries would vote an instant Yes on the question if Israel should be sanctioned.
In the end we need some form of "global democracy" with strong protections of individual rights.