The fact that the leaders of the Sunni minority oppose a federal structure for Iraq, and have the ability to torpedo the new constitution, does not change the reality on the ground that Iraq is already in effect three nations.
The Kurds in particular have both an effective local administration and by far the best militias to call on if needed. The Kurdish situation is also helped by the fact that it was really the Peshmerga who moved into the vacuum and liberated the Kurdish region whilst US and British forces smashed Saddam’s armies in the south.
Eventually if the Kurds do not get the autonomy they desire, it is just a matter of time before they simply secede and I rather doubt the US had either the stomach or the inclination to use force to prevent what is a purely internal matter for the Iraqis and Kurds. Any in any case, so what if Iraq breaks up? The obsession with ‘stability’ and countering Iran is what lead the West to unwisely back Saddam Hussain for so many years and look where that got everyone in the end.
An independent Kurdish Northern Iraq may give the Turks cause to fret (unfortunate but them’s the breaks) and give Iran acute dyspepsia (which has to be a good thing) because Kurdish success in Iraq will no doubt give the Kurdish minorities elsewhere ideas above their station. However I fail to see how thwarting long standing Kurdish aspirations is in the interests of the US and UK, particularly as the Kurds have been quite amenable to US interests as of late and have shown themselves to be the sharpest operators.
Of course the prospect of a Shi’ite Islamic Republic of Southern Iraq is not very agreeable but it at least has the virtue of allowing more tailored pressures to be put on the three constituent parts of ‘Iraq’ rather than a probably futile one-size-fits-all constitution which in any case may fall apart as soon as western forces pull out.
An Iraq of highly autonomous cantons is probably the best that Iraq’s Sunni politicos have any right to expect because the alternative is never going to be a return to the ‘good old days’ of Sunni dominance and centralised rule from Baghdad, it is going to be splitting the county in three independent parts. And there is something to be said for that anyway. To hell with ‘nation building’… sometimes the cause of liberty (and probably long term stability too) may be served by a bit of ‘nation wrecking’.