Can Iraq hold together? It's worth examining what is happening in that country through a broader prism. If you had looked at the Middle East 15 years ago, you would have seen a string of strikingly similar regimes — from Libya and Tunisia in the west to Syria and Iraq in the east. They were all dictatorships. They were all secular, in the sense that they did not derive their legitimacy from religious identity. Historically, they had all been supported by outside powers — first the British and French, then the superpowers — which meant that these rulers worried more about pleasing patrons abroad than currying favor at home. And they had secure borders.

There are exceptions. Algeria remains an old-fashioned secular dictatorship. Egypt — perhaps the longest functioning state in the world — has reasserted the old order by using force. The Gulf monarchies — Saudi Arabia, Kuwait, the United Arab Emirates — have withstood the turmoil partly because of greater legitimacy and mostly because of the massive patronage systems. And most hopefully, Morocco, Jordan and Tunisia have reformed enough to keep revolutionaries at bay.Today, across the region, from Libya to Syria, that structure of authority has collapsed and people are reaching for their older identities — Sunni, Shiite, Kurd. Sectarian groups — often Islamist in nature — have filled the power vacuum, spilling over borders and spreading violence. In Iraq and elsewhere, no amount of American military power can put Humpty Dumpty back together.

The old order was probably unsustainable. It rested on extreme suppression, which was producing extreme opposition movements, and on superpower patronage, which couldn't last over time. The countries with significant sectarian divides and in which minority groups ruled — Iraq and Syria — became the most vulnerable.

Let's be clear. The Iraq War was the crucial trigger and the American occupation needlessly exacerbated sectarian identities rather than building national ones. But once the old order broke, Iraq's Shiites, who had been suppressed for decades, in some cases brutally, were not likely to sign up to share power easily with their former tormentors.

It is true that during and immediately after the surge — 2007-08 — Prime Minister Nouri al-Maliki behaved differently. But if it took the danger of civil war, the presence of about 200,000 foreign troops, a particularly skilled American general (David Petraeus) and billions of dollars all to force him to make nice for a brief while, it was unlikely to be a long-term arrangement...