I remember watching this movie "Hamburger Hill" about the Vietnam War. I think the gist was that American soldiers had to continue retaking the same hill in an agonizing pattern of attack, occupy, retreat, attack, occupy, etc. Basically, the attackers aren't interested in the hill, they want to kill the insurgents. So when the insurgents (Vietcong or whatever) melt away, the hill is worthless.
Interesting article here about how Iraqi urban centers are becoming a similar case to Hamburger Hill. Apparently US forces have 'taken' Samarra 3 times in the last 18 months. Technically successful every time (because they occupied they're objective), they leave the area to fight elsewhere. In the meantime, the insurgents come out of hiding and re-occupy the urban center, with the support of the population.
Article even mentions insurgents bold enough to set up a roadblock on a main throughfare.
Warning: article is from a very leftist website, read with caution.
http://www.zmag.org/content/showarticle.cf...=15&ItemID=6363