Now let's make sure we win. But let's also make sure we don't lose too much in the fighting.

Because now, no matter how much damage we can inflict on our foes, we are vulnerable to horrendous casualties -- measured in the thousands today, maybe measured in the millions tomorrow. Put simply, the United States is too high-profile and too soft to survive intact a protracted 21st-century conflict. A look back at the history of warfare reminds us of how slow national commanders have been to realize the cost of being high, wide and handsome. But now, during this reign of terror, every American is on the front line of vulnerability and all of us must learn from the mistakes of the past. We're going to have to reconfigure ourselves, getting lower and harder.

In medieval times, the knight on horseback was dominant on the battlefield. Then the British came up with a devastating anti-knight weapon: the longbow, which could penetrate chain mail at 100 paces. At the battle of Crecy in 1346, and again at Agincourt in 1415, charging French knights were mowed down in rows.

Yet the use of horsemen in battle persisted for centuries, even as combat mortality rates soared. A man on horseback was much faster, to be sure, than a man on foot, and cavalry could always be used to terrorize the enemy. Polish knights routed the Turks outside Vienna, Austria, in 1683 wearing feathered caps that whistled in the wind, like birds of prey. But the main reason that the military style of large-scale cavalry formations lasted for so long was cultural: Officers and gentlemen liked riding around on horseback.

Other cultures were even slower to comprehend what was happening. At the Battle of the Pyramids in 1798, the Egyptian Mamelukes -- riding the finest Arabian stallions, wearing all their burnished armor, waving jeweled pistols and scimitars -- charged headlong into Napoleon's infantry and were slaughtered. Some 2,000 men on horses were killed. Perhaps 30 men on foot died.

As late as 1939, the Poles used cavalry against the mechanized Nazis; the result was heroic but tragic.

The unsolvable problem was that no amount of courage, and no amount of armor that a horse could carry, was sufficient to overcome the metalstorm reality of the modern battlefield. After centuries of increasingly steep losses, even the most gallant officers realized that they had to climb off their high horses and hole themselves in muddy trenches -- figuring it was better to be dirty than dead.

Today, to observe a battlefield is to see barely anything. That's the point: Anything that can be seen can be destroyed. So troops survive by burrowing, camouflaging and shrouding themselves in smoke. Even deep behind the lines, the military has similarly hardened and "stealthed" itself. And, while it's stupefying that a jetliner could crash itself into America's military headquarters, the explosion Tuesday posed no jeopardy to the Defense Department's command, control and communications systems.

Thus the paradox: American soldiers, bunkered up, are more likely to survive the next war than American civilians, out there all over the place. Indeed, our enemies know better than to go toe-to-toe with Norman Schwarzkopf. And so they come here in ones and two -- and see a whole country that is as vertical and vulnerable as equestrians at a horse show. So they don't need armies if they have nimble brains. They can flip airplanes into murderous judo, using the weight of the enemy against him.

Moreover, last Tuesday's disasters revealed new weaknesses. In high-tech offices, fires unleash so many toxic chemicals that the heroism of a burning-building rescue without a respirator is almost as dangerous as charging a fortress on horseback. And foes might be able to short-circuit the electronic grid that makes post-industrial life possible. Only Rep. Rob Andrews, D-N.J., and a few others have dared to think about protecting those pillow-soft systems.

The United States has no choice but to fight this war. But, during the fighting, we will likely discover that centuries of assumptions about how to behave, how to move and what to build will have to be unlearned.

Because, like cavalrymen sitting up there high and proud, countries, too, find themselves taking heavy casualties when they haven't been paying attention to what the enemy is up to.