US troops, says the study, should withdraw from Iraqi cities. This was "the only rational course of action, horrific though it will be", as America refocused its efforts from preventing civil war to containing its effects.
The unremittingly bleak document, drawing on the experience of civil wars in Lebanon, the former Yugoslavia, Congo and Afghanistan, also offers a remarkably stark assessment of Iraq's "spill-over" potential across the Persian Gulf region.
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In a "war game" testing US options, the Saban Centre for Middle East Policy at the Brookings Institution found that, as the descent into civil war gathered pace, confrontation between the US and Iran intensified, and Washington's leverage on Tehran diminished. Civil war in Iraq would turn Iran into "the unambiguous adversary" of the US.
Indeed, everything indicates that that is already happening. The study appeared on the same day as the Iranian ambassador in Iraq told The New York Times that Tehran intended to expand its influence in Iraq. US commanders now claim that thousands of Iranian advisers are arming and training Shia militias.
Of course, to the administration this is a feature not a bug.