I honestly think the UN needs to be changed so radically now, that the most effective measure would be to dissolve it and start over from scratch.
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It may be way too late for this type of approach. One of the major weaknesses of the U.N. is its membership rolls. Dictators, communists, thug totalitarian governments -- people that really have no interest in the original intended mission of the U.N. other than to be there in a transparent, miserable attempt to lend some form of credibility to their so-called countries.
Until this fundamental "country club of theives and rabble" gets modified, will you have a collection of truly interested parties in the mission of the U.N. -- otherwise just dissolve it, or at least throw it out of the U.S.A. for good.
Or perhaps we can just withhold "dues". Can't we use the old Steve Martin line when questioned about it, "We forgot"? And who will enforce our payment, the UN? Take away a large portion of their funding and they'll dry up. But you need to have the political will to withstand the certain backlash and I'm not sure our congress is willing or able. Now, if you get a feisty Ambassador who has nothing to lose and everything to gain, then that's a different story....