Next, the battle for Baghdad

The US military is on the verge of attempting something unprecedented in modern warfare: seizure of a sprawling capital without incurring heavy casualties, and without wholesale destruction of the city itself.

From World War II's battle of Berlin to the struggle for Hue in Vietnam, urban combat has proved a brutal, slogging business. That is the kind of fighting the Iraqi leadership now seems to be preparing for, as it hunkers down behind entrenched defenses and warns darkly of what will happen to US units in Baghdad's streets.

US commanders, for their part, have paused to pile up materiel for what may be a heavy blow intended to negate the need for house-by-house advance. Nor has the US given up on its preferred scenario: the collapse of Saddam Hussein's regime from within. "The US will try to make this unlike other [urban] battles," says Robert Pfaltzgraff, an expert on international security at Tufts University's Fletcher School of Law and Diplomacy. "The question is whether [it] will succeed in doing so."

At time of writing, the massed US Army and Marine units that have penetrated to within 50 miles of Baghdad had entered a phase commanders described as a "strategic pause."

Fuel, ammunition, and armored vehicles were flowing up from the south to US forces arrayed against Baghdad's outer ring of Republican Guards. In a way, the real war in Iraq seemed to be impending.

US operations to this point have involved fierce, but scattered firefights with pockets of Iraqi resistance, and Iraqi irregulars. Now, for the first time, the US-led invasion force is bumping up against a recognizable defensive front that stretches from the Tigris and Euphrates rivers near Baghdad eastward toward the frontier with Iran.

These Republican Guard troops may not have US levels of firepower, but they seem ready to fight, as shown when they repulsed a swarm of Apache attack helicopters with small-arms fire and other light weaponry.

The fact that Apaches attacked en masse - a force of 30 or so helicopters - suggests that the US may be switching the emphasis of its air campaign to close air support from the striking of strategic targets. At a briefing for reporters Tuesday in Kuwait City, US Central Command officials showed gun-camera footage of the destruction of a number of tanks and other heavy weapons, further bolstering this impression.

The US will move against Baghdad on its own schedule, said Central Command's Maj. Gen. Victor Renuart. But move it will.

"Baghdad is really the heart of the regime, and I would expect it would hold its most valuable treasures close to its heart," said General Renuart.

Not that Renuart, or any other US commander, is likely to relish that encounter. Cities are easy to turn into defensive redoubts, and the US military has little experience assaulting urban positions.

The most recent US urban combat experiences involve policing operations, such as in Haiti, Bosnia, and the early stages of the US involvement in Somalia, or raids to seize enemy leaders, such as in Panama and in the later days of America's Somalia experience.

Perhaps the most recent relevant example is Hue. US and South Vietnamese forces took three and a half weeks to drive North Vietnamese and Vietcong forces out of that central Vietnamese city, while incurring 147 US and 384 South Vietnamese fatalities.

But that was only a seven-block area, and involved nearly total destruction of the urban infrastructure.

Today, fighting in tight urban spaces would cancel out many US advantages in speed, knowledge, and smart weaponry, according to a 2002 US military doctrine publication on urban fights.

Tall buildings impede communications and use of satellite-guided bombs for close air support, for one thing. The enemy can't be engaged with smart weapons at long distance, for another.

Fire support in general is more difficult, as most artillery shells and air-to-ground weapons fall at angles too shallow to be effective. Controlling cities also requires large numbers of soldiers.

The solution to this problem for the US military? Perhaps it will be to try another style of fighting.

US commanders have described war plans that combine overwhelming fire power with deliberation. They will pick their targets carefully - presumably, they will aim for Iraqi leaders - and then hit hard, and quickly. This could mean fast-moving armored assaults, accompanied by helicopter or fixed-wing close air support, where possible.

The effectiveness of such an approach would depend on accurate intelligence, "so we can be precise in targeting those forces and leave urban populations basically unscathed," says retired Army Gen. John Reppert, a strategist at Harvard University's Kennedy School of Government.

That's one reason Special Forces have been in Baghdad for some days.

Another, slower, alternative to sustained urban combat, would be to establish a "loose cordon" around the city - blending elements of a siege with quick assaults and continued psychological war. But this could carry a toll on civilians.

Preferable, from the US view, would be for the regime to capitulate. From the first day of the war, when US cruise missiles rippled out in a "decapitation strike," the center of Iraqi power has been the key US target.

But aside from the continuing reports of Saddam's demise or injury, there is little obvious indication that the regime is cracking. "It doesn't matter if Saddam is alive or dead. Somebody is running a fairly vigorous and thus far competent defense," says Michael Corgan, a retired naval commander and professor of international relations at Boston University.

• Staff writer Seth Stern contributed to this report.