NATO is a defensive alliance created to defend it's members against aggression. It isn't an alliance that is designed to attack and occupy nations.
The U.N. has no troops. India and Russia have recently made it clear they would not send troops to operate under U.S. command. I don't think the Bush administration is ready or willing to turn the country over to the U.N. yet. That attitude might change if the press is reporting an American death every other day through November 2004. Those opposed to U.S. occupation of Iraq (of all stripes) surely aren't ignorant of that.