As a Rocky Mountain Institute report noted, the current situation echoes “shades of Civil War logistics, when mule teams hauled wagons of supplies, half of whose tonnage was feed for the mules.” These consumption patterns create huge operational demands across the tactical spectrum. Fuel delivery and supply line protection in the field require manpower that could otherwise be dedicated to combat operations; roughly one half of logistics tonnage for operations in places like Iraq is solely the movement of fuel.”
The above passage is taken from Fueling the "Balance": A Defense Energy Strategy Primer by P.W.Singer and Jerry Warner. The thirteen page document is an introductory text to the energy issues facing the Department of Defense and the military in general. A must read for those interested in the inter relationship of energy and war.
Where the issues of energy usage and logistics has always been an issue in the military, I think the issues has severely entered the calculus of military planners because of two things: the rising cost of energy and the type of wars that the United States is involved in.
The concept behind logistics is that a military needs a minimum amount of material to be able to operate in the field effectively. Logistics is the movement of those goods to that force in the field. Where there is movement there is energy consumption. The more energy that is consumed the more it generally costs.
For instance take logistics in Iraq involves moving fuel and items of necessity from Kuwait, on the highway, to Baghdad and elsewhere. Insurgents don't make the distinction between the front line and logistical train, in fact they attack where they feel they will be the most successful. Thus the improvised explosive device made for $350-$500 inflicts the cost of a truck and driver. Over time the cost of maintaining that logistical train becomes more and more expensive and the ability to maintain operations in that country become untenable.
Ultimately, in Iraq, it was the insurgent that imposed the greater logistical cost on the military not necessarily the ability to access the country. In Afghanistan you still see the costs imposed by the insurgents but the majority of logistical cost comes from the location of Afghanistan. The United States military risks having its energy cut off by not just hostile insurgents but also any number of surrounding countries that seek to leverage the United States.
All this factors greatly in the cost of operations in Afghanistan.