Why Marjah? The key is the geography of Afghanistan and the nature of the conflict itself. Most of Afghanistan is custom-made for a guerrilla war. Much of the country is mountainous, encouraging local identities and militias, as well as complicating the task of any foreign military force. The country’s aridity discourages dense population centers, making it very easy for irregular combatants to melt into the countryside. Afghanistan lacks navigable rivers or ports, drastically reducing the region’s likelihood of developing commerce. No commerce to tax means fewer resources to fund a meaningful government or military and encourages the smuggling of every good imaginable — and that smuggling provides the perfect funding for guerrillas.
via stratfor.com