Sounds good in theory.
Unfortunately, the U.N. as World Policeman runs aground on two reefs:
First, it's not run by people I would trust with that kind of power. Currently, the U.N. tends to sympathize, if not be run by, exactly the sorts of people who badly need a World Policeman to deal with them.
Second, it's pretty much a given that the U.S. will end up carrying the load anyway, both financially and militarily. Nobody else can do what we can do, even assuming they'd be willing to try.