American forces in Iraq are in danger of having their line of supply cut by guerrillas. Napoleon once said that "an army travels on its stomach." By that he meant that the problem of keeping an army supplied is the prerequisite for the very existence of the force.

A 21st-century military force "burns up" a tremendous volume of expendable supplies and continuously needs repairs to equipment as well as medical treatment. Without a plentiful and dependable source of fuel, food, and ammunition, a military force falters. First it stops moving, then it begins to starve, and eventually it becomes unable to resist the enemy.

In 1915, for example, this happened to British forces that had invaded Mesopotamia. A British-Indian force traveled up the line of the Tigris River, advancing to Kut, southeast of Baghdad. They became besieged there after their line of supply was cut along the river to the south. Some 11,000 troops ultimately surrendered, after the allies suffered another 23,000 casualties trying to rescue them.

American troops all over central and northern Iraq are supplied with fuel, food, and ammunition by truck convoy from a supply base hundreds of miles away in Kuwait. All but a small amount of our soldiers' supplies come into the country over roads that pass through the Shiite-dominated south of Iraq.

Until now the Shiite Arabs of Iraq have been told by their leaders to leave American forces alone. But an escalation of tensions between Iran and the US could change that overnight. Moreover, the ever-increasing violence of the civil war in Iraq can change the alignment of forces there unexpectedly.

Southern Iraq is thoroughly infiltrated by Iranian special operations forces working with Shiite militias, such as Moqtada al-Sadr's Mahdi Army and the Badr Brigades. Hostilities between Iran and the United States or a change in attitude toward US forces on the part of the Baghdad government could quickly turn the supply roads into a "shooting gallery" 400 to 800 miles long.

At present, the convoys of trucks supplying our forces in Iraq are driven by civilians – either South Asians or Turks. If the route is indeed turned into a shooting gallery, these civilian truck drivers would not persist or would require a heavier escort by the US military.

It might then be necessary to "fight" the trucks through ambushes on the roads. This is a daunting possibility. Trucks loaded with supplies are defenseless against many armaments, such as rocket-propelled grenades, small arms, and improvised explosive devices. A long, linear target such as a convoy of trucks is very hard to defend against irregulars operating in and around their own towns.

The volume of "throughput" would probably be seriously lessened in such a situation. A reduction in supplies would inevitably affect operational capability. This might lead to a downward spiral of potential against the insurgents and the militias. This would be very dangerous for our forces.

Are there alternatives to the present line of supply leading to Kuwait? There may be, but they are not immediately apparent.

A line of supply consists of the route and the facilities at both ends. Our present line of supply now originates in Kuwait with its ports, stevedores, warehouses, etc.

A new line of supply leading from Turkey or Jordan would require similar facilities. Turkey has not been very cooperative in this war, and a supply line leading from Jordan would have to pass through Anbar Province, the very heart of the Sunni Arab insurgencies. Creating new facilities in these countries would be possible but politically difficult, and it would take time.

Few of the permanent requirements for uninterrupted resupply can be satisfied out of the local economy. Iraq lacks reserves of these supplies, and there would not be anything like enough "left over" for our forces to subsist on.

What about air resupply? It appears that only 5 to 10 percent of day-to-day military deliveries into Iraq are currently transferred by air. Inside Iraq, local deliveries by air probably amount to more. In a difficult situation, the tonnages delivered could be increased, but given the bulk in weight and volume of the needed supplies, it seems unlikely that air resupply could exceed 25 percent of daily requirements. This would not be enough to sustain the force.

Compounding the looming menace of the Kuwait-based line of supply is the route followed by the cargo ships en route to Kuwait. Geography dictates that the ships all pass through the Strait of Hormuz and then proceed to the ports at the other end of the Gulf. Those who are familiar with the record of Iran's efforts against Kuwaiti shipping in the Iran-Iraq War will be concerned about this maritime vulnerability.

Potential adversaries along the line of supply include many combat-experienced and well-schooled officers and former officers. We can be sure that they are acutely aware of this weakness in our situation.

The precarious nature of our supply line is well-known to our military leadership. Unfortunately, this is one of the many problems in Iraq that has not been adequately addressed because of a shortage of troops. We should start building ourselves another line of supply as a backup, and we should do it soon.

• Patrick Lang is former head of human intelligence collection and Middle East intelligence at the Defense Intelligence Agency.