No, the UN is not a government of any sort. It is a forum for countries to sort out disagreements, and make agreements. The UN charter is clear in that the UN itself shall always respect a countrys sovereignty. And that is, I believe, a big reason why the UN wouldn't ever support an invasion such as the invasion of Iraq. And I also believe that is also why Kofi Annan claimed the "war" illegal, as it went against the agreements made with the UN, and between the countries that are members of it.