Debra Schifrin, reporting on NPR about the history of walls (Dec. 30, 2003):

In the first century AD, the Roman emperor Hadrian built a massive stone wall dividing England and Scotland . British archaeologist Nick Hodgson says Hadrian built the wall to keep marauding bands in the north out of the Roman province of England .

Mr. NICK HODGSON (British Archaeologist): If the wall had not been there, such groups would have been able to penetrate the province very, very rapidly. The wall was there to allow the army to do something about their presence.

SCHIFRIN: While the wall succeeded in protecting the English villages to the south, Hodgson says, it made life very difficult for the people to the north of the wall.

Mr. HODGSON: They were cast out of the Roman Empire once the wall had been arbitrarily cut through the island of Britain and cut a good deal of the population off.

SCHIFRIN: Two hundred and fifty years later, Hadrian's wall was overrun by forces from the north. In fact, all the great walls were eventually overrun, according to Jonathan Roth, professor of military history at San Jose State University . Even the Great Wall of China , built up over a millennium to its peak of 4,000 miles long, was overrun several times by the Turks, Mongols and the Manchus. But there is a dispute among historians about the main role of these walls. Many historians like Roth now argue they were not built primarily for defense, but for economic functions.

Professor JONATHAN ROTH ( San Jose State University ): They were enormously expensive to build and enormously expensive to man and relatively ineffective in keeping enemies out. What they could do is prevent wagons from crossing, and that's the sort of thing you would collect customs duties.

SCHIFRIN: Enough customs duties, Roth says, to pay for the wall and then some. In that sense, he says, the walls served their purpose well. If the fence was only a secondary role of giant walls in ancient world, Roth says, it became even less important in walls built in the second half of the 20th century. The invention of airplanes, tanks and high explosives made walls much less effective as military barriers. Roth points to the Berlin Wall, which the Soviets built around West Berlin in 1961. Its purpose was to stop the thousands of East Germans who were crossing into West Germany . Roth says the wall was flimsy.

Prof. ROTH: You could take a sledgehammer really, as people did, and knock it down. It's a barrier to stop civilians from crossing. It's not intended to stop soldiers and armies and tanks.

SCHIFRIN: But some civilian East Germans did find a way to get past the wall. That has also been true for Mexicans illegally crossing the steel and metal fences that line the US-Mexican border. Belinda Reyes, a research fellow at the Public Policy Institute of California, says one illegal immigrant made this analogy.

Ms. BELINDA REYES (Public Policy Institute of California): If you look at a cow, and the cow eats the grass on this particular place and there's no more grass, even though there's a fence, if there's grass on the other side, they're going to jump over the fence and eat the grass on the other side. So he was, like, 'Are we going to be stupider than the cow?'

SCHIFRIN: Reyes says while the US is failing in its stated goal of stopping illegal immigration, it is succeeding in its symbolic goal: to show Mexicans and Americans that something is being done. Ironically, as we move into the 21st century, military historian Jonathan Roth says, walls are regaining a military function, because instead of fighting armies, countries are now fighting civilians. They are fighting terrorists.

Prof. ROTH: Tanks and planes don't really defend against terrorists, whereas barbed wire and concrete walls can.

SCHIFRIN: The great walls in history have been relatively rare because few empires or countries had the economic power and military strength to build them. But when an empire builds a wall, it often foreshadows the empire's eventual decline. Archaeologist Nick Hodgson says, for instance, that the great Roman walls kept attackers out, but they also kept the Romans in.

Mr. HODGSON: It really meant the end for the expansion of the empire. The forward-pushing Roman world of the republican early empire disappeared. And it became very much a static item, the Roman Empire .

SCHIFRIN: The word 'limit' comes from the Latin word 'limes,' which means 'border' or 'boundary.' History may be teaching us that although walls can symbolize the greatness and strength of a civilization, they can also symbolize its limits.