Here's Larry C. Johnson, quoted in part of a longer piece on the same theme at James Wolcott's Blog:
The ongoing attacks on bridges in and around Baghdad creates significant risks and logistical obstacles for U.S. forces in Iraq. In my opinion these attacks are part of deliberate strategy to create ambush chokepoints, degrade the capability of U.S. Quick Reaction Forces, and enhance the ability of insurgent forces to cut the U.S. lines of communication.
[T]he destruction of bridges can produce the defacto isolation of U.S. outposts and bases. If a U.S. unit is attacked and requires reinforcements, the loss of these bridges increase the difficulty of the U.S. Quick Reaction Force reaching the scene in a timely manner. Moreover, with fewer alternate routes available, insurgents can anticipate where to hit a responding American force.
Every day they stay there, American soldiers are at greater risk, to no sensible purpose. We've reached the point now that leaving (imagine convoys evacuating over an ever-dwindling number of bridges?) will be as dangerous as staying, perhaps more so.
It's a given that Republicans will attempt to blame the Democrats for this ghastly result of their own single-minded foreign policy. That's one of only about three tricks they still have in their bag--blaming others for their own failures. The only question is, will anyone take them seriously?