What happens if they don't agree with the US on something?
Or support pro-choice policies?
Or ban farm subsidies in the name of "free trade"?
What if the UN is thought to be working for most of these places, except for a rogue US?
In theory it's a GREAT idea, I believe, and in practice I think the US would be impatient with anything which didn't just rubber-stamp it's policies. And I doubt if there are many nations which would trust the US to act cooperatively.
Consider the Iraqi weapons inspections.
Arf