Military historians call it the difference between "the teeth" and "the tail" -- an army's front line troops and the suppliers who fuel them.
As U.S. ground forces press rapidly and farther into Iraq, trailing behind are mechanics, cooks, doctors and drivers -- many of them vulnerable, like the 12 military personnel missing or captured over the weekend.
Pentagon officials would not identify the unit, but some of those captured by Iraqis were from the 507th Maintenance, part of the 111th Air Defense Artillery Brigade. The 507th, mostly mechanics, keep the diesel tanker trucks rolling, fix generators and keep the mechanical parts in good shape.
Five American POWs, at least two of whom said they were with the 507th, were questioned on Iraqi television over the weekend, with footage that also showed what appeared to be four corpses. Another two Americans were captured Monday when their helicopter went down.
"It's not a new thing," said Professor Roger Beaumont, who teaches military history at Texas A&M. "You have a network essentially behind a modern army ... like a spider web."
Alexander the Great kept supplies close at hand; Napoleon famously remarked that an army travels on its stomach; the eastern front in World War II, opened when Nazi Germany invaded Russia, became a nightmare of sniping attacks on supply lines.
In Iraq, troops are moving rapidly into hostile territory. At the same time, the supplies needed to fuel them are potentially open to attack by those loyal to Saddam Hussein, who may be part of -- or hidden among -- the general population.
The United States' emphasis on speed and firepower, combined with several goals set out by the Americans, combine to raise new questions that will play out over the course of the war, several historians and analysts said.
"The mixture of missions given to the military has in mind not only winning the war, but also winning the peace at the same time," said Arthur Helton, director of Peace and Conflict Studies at the Council on Foreign Relations.
"How challenging for the United States to be involved in war fighting, protecting its own troops, and at the same time administering to needy Iraqi civilians.
"That's a challenge that I'm not sure if any military can adequately discharge."
There are the usual needs for fuel, food, ammunition and medical care. Then there are the additional demands of caring for potentially large numbers of Iraqi POWs, and trying to assist the civilian population.
Given U.S. command of sea and air, however, there's not much the Iraqis can do to disrupt the supply line, said Miguel Centeno, a Princeton University sociology professor who is an expert on history of war. Nothing short of a massive missile attack could interrupt the caravan.
"From what I know, the Iraqis don't have the capacity to do that. Even a terrorist attack on the scale of 9-11, as horrible as it would be, would not interrupt the logistics of it," he said.
The stronger a military force is, the greater its reliance on supplies -- hence the long-recognized tension between "the tail" and "the teeth," Beaumont said.
In World War I, there were roughly six supply personnel for one fighting person; by World War II, the ratio had grown to 10 "tails" for every "tooth"; by Vietnam, it was up to 13 to 1, Beaumont said.