Maybe he's not going to war, maybe it's all a bluff. Or maybe they are going to do something very stupid and very reckless. If you could rely on governments not to do things just because they were very stupid and very reckless, we would have relatively few wars.

If the USA does go to war unilaterally or virtually unilaterally, what I imagine they'd do would be to use aircraft carriers and bases in neighbouring countries (regardless of what the inhabitants or even the governments wanted) to bomb Iraq from a great height, either selectively or massively, in the expectation of a collapse of civil society and of any significant military opposition. The idea then would be to send in some kind of ground troops.

Maybe Turkey could be brought on side to occupy Northern Iraq, and crush any Kurds who wanted to preserve the limited autonomy they have at present.

And there'd be an effort to have some kind of Iraqi opposition involvement, ideally to provide ground forces, but at any rate to provide a political figleaf.

And with luck there'd be some Iraqi general on the same lines as America's former protegé Saddam to come over, and emerge as the new strong man in Iraq.

Whether it all works is pretty questionable. I think the chances are it would all end up with a radical Islamic regime in power in Iraq and Saudi Arabia and a few of the Gulf States. And a lot more potential for death and glory operations like September 11.