By midmorning Sept. 11, New York City Police Commissioner Bernard Kerik has recalled, he realized his city had suffered an aerial attack, "and I don't have an air force." Someone had to close down the airspace over New York. "We have to call for air support," he thought, "and you know, is there a number to do that?"

Strategists at the Pentagon have known a long time that urban warfare would be central to 21st-century battles. What they weren't prepared for was to chart them over the Atlantic Seaboard of the United States.

When it became clear that New York City was under terrorist attack, commanders meeting in the Pentagon began giving orders for a military response even as American Airlines Flight 77 slammed into their building. By 9:25 the FAA consulting with the Pentagon banned all takeoffs and ordered the airspace over the United States closed. "When it became clear what the threat was, we did scramble fighter aircraft, AWACS, radar aircraft, and tanker aircraft to begin to establish orbits in case other aircraft showed up," said Gen. Richard Myers, chairman of the Joint Chiefs of Staff.

Those patrols continued between New York and Washington for weeks after the attack. Aiding U.S. air forces were British AWACS operators and other NATO forces in the alliance's first joint defense of the continental United States.

Joint warfighting and agility in the skies are critical to fighting over urban areas. Why they are increasingly important isn't only about Sept. 11. It's also a matter of simple demographics. Virtually all the population growth expected in the next 20 years will take place in urban centers. Nearly 2 billion people will be added to the developing world by 2030. In 1990 cities with populations between 1 million and 10 million numbered 270. By 2015 they will number 516. Thriving in those conditions are the kinds of young, discontented men who turned up in al-Qaeda terrorist camps.

The haphazard growth patterns that will rule in these cities pose a threat to fighting forces. The slums of Karachi or New Delhi or Cairo are uncharted sprawl with no connection to traditional infrastructures, and no blueprints. Their mapless density increases the risks for U.S. forces, who are more and more dependent on remote target sensing and satellite images. Even within established gridlines accuracy is crucial, as U.S. fighters found when they mistakenly bombed the Chinese embassy in Belgrade during the war against Yugoslavia.

Recent history teaches the importance of being able to maneuver above and within cityscapes. In the last decade U.S. forces were notably engaged in Kuwait City, Mogadishu, Panama City, Port-au-Prince, and-most recently-the Afghan cities of Mazar-i-Sharif and Kabul.

Eighteen U.S. Marines at the mercy of slumlords in Somalia's capital, Mogadishu, perished when what began as a relief operation descended into urban combat in 1993. For U.S. forces it was an important lesson: Operations that look like humanitarian projects or rescue can quickly deteriorate into full-pitch battle.

It also taught U.S. forces that urban warfare is not three-dimensional but five. They have to pay attention to the man-made surfaces like rooftops and alleyways-what strategists call "supersurface"-and the subterranean levels of city battlegrounds. In the war on terrorism, strategists expect increasingly that ground troops and air forces will face unconventional foes in cities that have these modern features but are at the same time and in some ways more primitive.

Fighters know they cannot destroy the city in order to save it. Surgical strikes in Baghdad during the Gulf War, says Air Force Capt. Troy Thomas, proved that air power, properly executed, can reduce civilian casualties and collateral damage. New gadgets aren't as likely to hone those skills as will better intelligence and carefully crafted rules of engagement.

Anticipating that those skills will be called into use soon, U.S. Marines launched Project Metropolis late last year. Strategists hope to improve reconnaissance tactics, sneaking troops into Mogadishu, for example, to train and collect intelligence without detection. Under the project, Marines are trying out new tactics in U.S. cities: Little Rock, Ark.; Chicago; and Boise, Idaho. This has the advantage of practicing domestic security and coordination with local law enforcement while practicing new warfighting techniques. Other forces may soon join Project Metropolis.

At the Marine Corps Warfighting Laboratory in Quantico, Va., retired officers were brought in after Sept. 11 to strategize alongside active-duty counterparts. The Marine Corps has already made changes in its cold-weather training program as a result of war games acted out with Afghanistan in mind. The group also looks at war scenarios in hot urban climates like the Philippines. Longer-term strategies for the war on terrorism are tested under Project O'Bannon, named for a noted 19th-century Marine who fought Barbary pirates.