There have been a lot of rumors recently about US supply line troubles in Iraq. I've been scouring the news, but there was nothing until this morning. Now it comes out: there are
two separate supply line problems. One is in Fallujah, where marines have been cut off from Baghdad. Insurgents control the 30-mile stretch of road behind them. The other, possibly more ominous problem is in the South. There are only a few routes from Baghdad to the Persian Gulf. From
this map it appears to be possible to cut off all routes by controlling just two cities, Nasiriyah and Kut. In fact, the US has been battling for control of these two cities over the course of the last few days.
If the southern supply lines are cut, troops in Baghdad will be cut off and isolated. I'm not sure how grave the consequences would be. Theoretically, troops in central Iraq could still be supplied from the north, via Turkey, or by airlift. But I don't think the US military is prepared to supply them that way. At the very least it would be an enormous hassle, requiring months of restructuring.