An excellent response Tony, thank you
The UN might also be able to help establish an outside influence in Iraq by non-military means. At some point we (the world, that is) have to try to establish working relationships with the powers-to-be in Iraq (not all of which may be handpicked by the US) that doesn't involve holding a gun to everyone's head.
I haven't heard anyone say the UN would be a cure-all, but a UN presence could carry some serious weight, especially if the US would quit undermining it at every turn. It would be indicative of a truly concerted effort on the part of much of the world.