A number of bridges in Iraq have been blown up recently.

Suspected Sunni insurgents bombed and badly damaged a span over the main north-south highway leading from Baghdad on Tuesday — the third bridge attack in as many days in an apparent campaign against key transportation arteries.

There are signs this could be the beginning of a larger assault against US forces and the Iraqi government. It crimps the supply lines, makes it harder to bring in reinforcements and concentrates traffic on remaining routes.
In Bridge Bombing Campaign Escalates Further, the blog Gorilla’s Guides has details on the bridge attacks.

The bombings outside of Baghdad are aimed at rendering the country ungovernorable by denying huge swathes of territory to the American forces. Good example of attacks aimed to achieve this are the Sahra bridge bombing and today’s Al Sabtiya bridge bombing.

Larry Johnson at No Quarter has a post, The Bombed Bridges of Baghdad writes,

The systematic destruction of bridges in and around Baghdad are the early warning signs that the mission for our soldiers in Iraq is going to get tougher and more deadly.