Arming the Sunni tribal and militia forces could accelerate the division of Iraq into mini-states, if the United States ends up arming all three sides in the Iraqi civil war. That seems to be what we're doing. In a new report, Tony Cordesman of CSIS comments on the downside of building up the Sunni forces:

Nevertheless, the net result, however, is to create a separate Sunni or Shi’ite force whose ties to the central government are uncertain and opportunistic. It is also to create a force built on uncertain tribal and local coalitions. This makes it far from clear what kind of political power such forces will support. For example, in Anbar, it might create the core of a more effective national Sunni political party or role in the central government. It also, however, might emerge as a regional tribal political force that challenged the government or became a new source of armed opposition to it.

Anbar will not be a model for the rest of Iraq, but it exemplifies a growing reliance on local forces may well be the coming paradigm. The continuing failure of the central government’s effort to develop an effective police force, and one that can “hold” the “wins” of Coalition forces and the Iraqi Army is leading to more and more reliance on sectarian and ethnic local security forces in other areas, or to reliance on local police under the de facto control of local political leaders. Almost all of these local political groups and forces are divided along sectarian and ethnic lines.