The US just uses the UN for cover. Or at least it tries to. In the case of invading Iraq, there was no UN mandate for doing so. But after it invaded, it got the mandate because the US had already broken it and needed to fix it. When the UN mandate for the US to be in Iraq expires, the US will no longer have any cover for its presence there. This will make it a lot more difficult for the US government to pretend it's there for any reason having to do with what's good for the Iraqis. That could make it harder for the government to get enough support from voters and congress to be able to retain a military presence in Iraq. We'll see.