Christopher Bellamy: If the cities do not fall to the Allies, there may be no alternative to siege warfare

Four centuries before Christ, China's General Sun-Tzu wrote in his Art of War: "The worst policy is to besiege cities." Nearly two and a half thousand years later, the Americans and British invading Iraq and trapped outside Baghdad and Basra have again become aware of this.

Six months ago, on the first anniversary of the attacks on New York, Tariq Aziz gave an interview to Dr Toby Dodge, of the University of Warwick. Mr Aziz said: "People say to me, you [the Iraqis] are not the Vietnamese. You have no jungles and swamps to hide in. I reply, let our cities be our swamps and our buildings our jungles."

This interview is cited in Dr Dodge's perspicacious and prescient article, Cake Walk, Coup or Urbanwarfare: the Battle for Iraq, in the Adelphi Paper No 354, Iraq at the Crossroads, published by the International Institute for Strategic Studies. Maybe, this time, the academics got it right.

So the Iraqi plan to frustrate and hold the Americans was clearly well thought out six months ago. As Thomas Ricks, a Washington Post reporter wrote yesterday, the unexpectedly bad weather, long and insecure supply lines stretching 300 miles and surprising Iraqi resistance have led to a "broad reassessment of timelines". In other words, a longer and harder war than was expected a week ago.

An operational pause is in the offing. The 3rd US Infantry Division (Mechanised) is a relatively small force (though more than double the size of even the best-appointed Iraqi division), more than 300 miles from its base. Its 100 Apache helicopters, its main hi-tech, anti-armour assets, are grounded by sandstorms. It is even having emergency supplies of water and food trucked in from Kuwait.

The logistics people in their soft-skinned vehicles are probably even braver than the frontline soldiers. And they are the people the Iraqis, striking at the vulnerable supply lines, will go for. Senior US officers were cited as saying the 3rd Division must "run out of steam soon". A sobering thought, if true.

The US and British commanders must be concerned about the cities they never wanted to besiege. Baghdad has a population of five million. Najaf, the main jumping-off point for an attack on the capital, has 600,000. Basra has 1.5 million. That is the same population as Northern Ireland. The British have still not brought total peace to Northern Ireland after having deployed, on average, 17,000 troops there, for nearly 34 years. If the main Iraqi cities do not come over to the Allies, and so far they have not, we face a very different strategic problemto the one we faced on the outbreak of war more than a week ago.

And the south-central Iraqi theatre will be the operational area of the US 4th Infantry Division (Mechanised). Its heavy equipment is in 35 ships heading for Kuwait from the Mediterranean via the Suez. The division, originally intended to strike Iraq's northern front through Turkey, will not be be ready until April.

In southern Iraq, where resistance has been far tougher than expected, its appearance will, nevertheless, be hugely welcome. As one retired US general said: "I wouldn't like to go into Baghdad before I had another division up into my rear." That can only be 4th Division, and it will not be there until next month.

In the interim, 3rd Infantry's long and vulnerable supply lines, attacked by Iraqi stay-behind parties, can be reinforced only by the 82nd Airborne Division, based near Kuwait city, and the 101st, who are, to quote The Washington Post, "deep inside Iraq". Part of one US airborne brigade, the 173rd, dropped to hold an airfield an hour's drive north of Irbil on Wednesday night.

Quite why the Americans felt they had to drop 1,000 men by parachute 50 miles behind the front line in safe, secure, Kurdish territory to an airfield on which they were already landing helicopters is unclear.

It could only have impressed the Western media. There is no way it could have impressed Iraqis. The British Paras don't parachute if there is an airfield to land on, and some of them yesterday were not impressed, either. But it made good TV in the absence of much other good news.

Over the past 24 hours, the overwhelming lesson has, again, been: be sceptical. On Wednesday night and Thursday morning there were reports of a huge Iraqi armoured force – 1,000 vehicles, about a brigade – trying to cut off the vulnerable US main supply route, and of the breakout of an Iraqi battle group – 120 tanks – from Basra down into the Al-Faw peninsula.

In the cold light of dawn, the great brigade counter-sweep dissolved. The battle group was shot up by well-prepared US and British armour, artillery and air power.

Christopher Bellamy is professor of military science and doctrine at Cranfield University