What happens if Saddam fights back with chemical and biological warfare?

London and Washington insist the Iraqi dictator has weapons of mass destruction. If so, war planners

The UK has been mesmerised by the terrorist threat posed by chemical and biological weapons at home. Much more imminent and likely, however, is the use of these weapons on the battlefield - possibly against British troops.

The BBC broke a story that Baghdad is equipping the elite Republican Guard units with protection against chemical weapons. Apparently, new chemical warfare suits have been issued, as have supplies of atropine, a drug to counter the effects of some chemical agents. The information, provided by external Iraqi opposition groups, is said to have been smuggled out of Iraq recently. Many of these groups have been funded by the Americans, so the timing of the news may be a little suspect. But there is little doubt that the Iraqi army is ready and able to use biological and chemical weapons.

Saddam Hussein knows that the Anglo-American forces will not deploy chemical or biological weapons against his forces - although there is the increasingly explicit deterrent threat that Washington will use nuclear weapons if Saddam resorts to large-scale use of unconventional weapons against western troops. Also, the Secretary of State for Defence, Geoff Hoon, has again said that the UK government "reserves the right" to use nukes in extreme circumstances. Saddam has frequently ignored or misread such western signals and may be planning to use chemical or biological weapons in extremis, in the so-called Samson option.

Defence against such weapons is a pressing issue, then, in the US and UK military establishments alike. Despite improvements in technology, it is a facet of military thinking that has remained largely unchanged since the cold war. Soviet forces spent much time and money developing their ability to use CBWs (chemical and biological weapons) against Nato. CBW attacks were meant to shape the battlefield in much the same way as a minefield would. Opponents would be forced either to sit tight and wait for the chemical attack to pass, or to bypass the affected area. Either way, it was a tactic that allowed the enemy to try to direct the tempo and direction of the battle.

Counter-attacking through a CBW assault was considered so arduous - due to the rigours of the protective equipment - that it was almost entirely discounted. The British army, for example, has a policy that, unless specifically ordered, attacking in full IPE (individual protection equipment, required for chemical weapons) is seriously to be avoided.

Would this traditional military practice apply in a war with Iraq? If Saddam uses CBWs in the desert, he could be playing to the strengths of the allied forces. Chemical agent clouds work both ways, limiting the perpetrator as much as the intended victim. They are likely to be less of a limitation to the technologically advanced Anglo-American forces than to the Iraqi troops.

What if Saddam delays using his doomsday weapons until the allied forces are in the sprawling outer suburbs of Baghdad?

Urban warfare, such as in Stalingrad, is a difficult task; when you combine it with CBW it becomes almost impossible. The effects of a CBW attack in a city can cover a far wider area, and last a great deal longer, than a CBW onslaught on desert battlefields. In modern cities, wide roads and well-planned geometric areas increase the airflow, speeding up dispersion. Older, irregular cities, or those with cluttered areas such as souks, encourage a wider dispersion but with a far longer "linger" time. Some parts of Baghdad, those near to the Tigris or the expressway, for example, may well permit a rapid dispersion. Areas that have few wide streets, with narrow bazaars, or that may have been reduced to rubble by bombing, will slow the airflow and could hold enough agents in the atmosphere to last many hours. If this is combined with a persistent agent, such as VX, some districts may be contaminated for many weeks.

This level of contamination would set off the allies' chemical and biological detectors, and attacks would either have to be stalled or forced into other areas. This means contact with the enemy is lost, tempo is slowed and the enemy gains a respite and time to prepare for the next attack - not things you want to encourage in urban warfare. It also forces troops into full IPE, and in a hot environment this is especially draining. Troops are recommended to stay no longer than eight hours in IPE; in a very warm climate - Iraq after March - this decreases rapidly, so troops have to be recycled constantly. "Clean" (uncontaminated) areas need to be set up, and troops carefully monitored. This type of "refreshing" is difficult enough at the best of times, but a constant stream of troops in both directions would make a prime target for enemy snipers.

Chemical weapons were never seriously considered as part of urban defensive strategy during the cold war because of the impact on civilians. Once released, an agent is indiscriminate, and can penetrate most buildings through air-conditioning, or poor seals on windows and doors. But what if "collateral damage" - the death and incapacitation of civilians - becomes an asset to a ruthless tyrant? It is impractical to hope that all civilians within the conflict area will have left their homes; so, if CBW were used, Iraqi civilians would be affected. If sarin or other nerve agents were deployed, the loss of civilian life would be high.

What happens if blister agents such as mustard gas are used? Surviving civilians would be forced to leave their refuges and take to the streets, further bogging down the allied advance. The melee of combatants and civilians could create enormous humanitarian problems for the allies and drain precious medical resources.

Yet this is not the worst scenario. If a biological agent were released, civilians would have to be found and forced out of every house and cellar. Any left behind would be a threat to the military "dirty line" (the line behind which everyone is "clean" or uncontaminated); this could lead to recontamination and a series of outbreaks in the rear area. The surge of war would also drive infected civilians out of the war zone, through both front lines, into other areas of the country and, even worse, to refugee camps - which would be ideal centres for incubation. That could be the start of a number of epidemics, turning into pandemics, which would recognise no borders.

This mix of military and civilian problems is the worst scenario: an Iraq populated by the sick and dying, with no infrastructure, and hampered by continual outbreaks of ebola or anthrax - some areas of Manchuria are still suffering from outbreaks initiated by the Japanese Unit 731 in the Second World War. The departure date for any allied peacekeepers would be far distant.

Until recently, this scenario would have been far-fetched - what leader would kill his own people in order to hang on to power for a few more weeks? Now this forecast does not seem so fantastic, and British and American politicians are yet to come up with a response to it. The best that they can hope for is damage limitation and draconian laws on movement.

The Pentagon's strategy is founded on a quick end to the war, based on a palace coup against Saddam. But, cornered, he may fight back - with all his CBW stockpiles. Possibly the best hope of stopping him rests on the consciences of the officers asked to release the agents that could kill thousands of people. Then the question the Pentagon generals have to ask themselves is: "Can I trust Saddam's Republican Guard?" Inshallah.

Paul Moorcraft and Gwyn Winfield work for Defence Review