"The main lesson of the invasion of Iraq is that you cannot engineer society," says Mr. Brom.
Sectarian competition will continue
Today, years of internecine warfare have taken their toll. Even under the best of circumstances, it appears inevitable that Sunnis, Shiites, and Kurds will compete for political and economic influence for years to come.
Within those sectarian and ethnic groupings various factions will vie for influence as well. At issue will be the nature of competition: ballots or bombs?
"People aren't going to be reconciled, there has been too much violence.... the question is can they work out a new set of arrangements," said Anthony Cordesman, a military expert at the Center for Strategic and International Studies, in a February briefing.
At this point in its experience in Iraq, the US is no longer trying to win a great victory, according to a range of analysts from around the world. It is simply trying to avoid defeat.
Much depends on the attitudes of the various groups toward basic questions of national identity. But even after all this time, the US may not really know whether the Kurds want their own state, whether Shiites will allow true Sunni participation in central government, and whether the Sunnis – Iraq's old ruling class – truly have given up dreams of reconquering the country.
The most crucial unknown: Is the current decline in sectarian violence a real trend – or are Sunnis and Shiites simply rearming and biding their time?