If this is a new kind of conflict, what shape will it take?

The US military is about to engage in a conflict that will be diffuse, dangerous and very protracted. Not only is there no guarantee of victory, but it is hard even to know what victory would look like.

The US military is about to engage in a conflict that will be diffuse, dangerous and very protracted. Not only is there no guarantee of victory, but it is hard even to know what victory would look like.

In some respects, it will be a new kind of war. In others, it will look very much like older conflicts, especially the small wars fought by Britain as it defended and then retreated from Empire.

Military theorists call it a "fourth generation war", a messy conflict in which it is hard to identify the enemy, find the battlefield or define success. It will be different to the large-scale engagements that have dominated American military thinking since 1939; but also to the techno-war for which Washington has been planning.

US forces have been training, for 50 years, for large-scale encounters involving very heavy weapons, including nuclear armaments. The success of American military thinking as well as technology seemed to be confirmed by its crushing victory in the 1990-91 Gulf War, achieved through a massive commitment of aircraft, tanks and troops.

The US has tried to capitalise on its strengths. The most influential idea in US military circles for the past decade has been the so-called "Revolution in Military Affairs", spearheaded by Pentagon intellectual Andrew Marshall (no relation). This focuses on the power of high technology, and has led the Pentagon to complex new ideas that blend war, information and management theory into what it calls its "Joint Vision".

The RMA envisaged war fought through computers and satellites more than guns and trenches. It took Gulf War precision bombing and information technology and projected them into a future where machines and information, not men, dominated the battlefield. It was, for the large defence contractors and the budget warriors of the Pentagon, an attractive idea.

Even before the present conflict, military analysts had warned that the Gulf War was deceptive and things might look very different. The Gulf War "seemed to work out okay for us, but ultimately it may be an aberration", said former US General Anthony Zinni in a speech last year, "because it may have left the impression that the terrible mess that awaits us abroad... can somehow be overcome by good, clean soldiering, just like in World War II."

Indeed, he added: "The only reason Desert Storm worked was because we managed to go up against the only jerk on the planet who was actually stupid enough to confront us symmetrically."

Other military experts had started to sketch out an idea of something much more complex, something that looks more like the conflict which the US faces today in the hills of Afghanistan and around the world.

An influential article written 12 years ago for the Marine Corps Gazette first sketched out an alternative, based on the rise of new, transnational threats to American security like terrorism. Fourth-generation warfare would be completely different from the models that preceded it, the authors said.

The battlefield would be dispersed and include potentially the whole of society, not a discrete terrain like the deserts of Iraq. Small, manoeuvrable, agile forces would dominate, not the heavy armoured forces that the US assembled in Saudi Arabia.

The goal would be to collapse the enemy internally rather than destroy him on the battlefield. And, it noted, it wouldn't necessarily be the mighty industrial and technological war to which America had become accustomed. But then that was the point.

"A fourth generation may emerge from non-Western cultural traditions, such as Islamic or Asiatic traditions," it said. "The fact that some non-Western areas, such as the Islamic world, are not strong in technology may lead them to develop a fourth generation through ideas rather than technology."

Aware of the threat, the Pentagon has started to focus on what it calls asymmetric warfare. This buzzphrase refers to conflicts when an enemy attacks where it knows America is weak.

But its fears have primarily been of attack by "rogue states" using missiles with chemical, biological and nuclear weapons; the mooted defences have been anti-ballistic missile defences. Instead, the weapon was US civilian airliners.

A fourth generation war goes against the grain of the highly organised, technological focus of the US military establishment. "Roughly speaking, 'fourth generation warfare' includes all forms of conflict where the other side refuses to stand up and fight fair," wrote one military analyst.

The battlefield for the coming conflict is, in essence, everywhere. If the US is right to believe it faces a largescale confederation of groups, they may have sleeper cells anywhere. New recruits can be found quickly, and training doesn't need to be intense. The targets may be in Kabul or Croydon.

The conflict will probably be more like the post-colonial or counter-insurgency wars that Britain fought for the last century than the Gulf War. Indeed, the US Army Armor Center at Fort Knox still includes, on a list of suggested reading, Col CE Callwell's Small Wars: A Tactical Textbook For Imperial Soldiers, first published in 1906.

Britain fought wars in Iraq, Sudan, what is now Yemen and Afghanistan, all putative battlefields now, as well as Kenya, Malaya and Cyprus. Many of these were failures. But Britain drew a number of lessons that helped it score a success in Oman, for example, where British forces helped crush an insurrection in the 1970s.

Militarily, Britain used a mixture of air power and irregular or special forces, often with local units. Punitive expeditions by aircraft were matched by small patrols of special forces, aimed at ambushing one camp, one vehicle or perhaps even just one individual. Soldiers might spend a week crawling a hundred yards to shoot one individual. Mobility, intelligence and adaptability were key.

But tactics weren't the point. The main lesson that British forces drew from both success and failure was not military: it was political.

Britain realised that counter-insurgency warfare, like the conflict that the US now proposes to fight, involves welding together political, military, judicial, social and economic dimensions, and integrating each into a unified strategy. Rather than beating the enemy on the battlefield, it focused on beating them socially, politically and psychologically.

The darker side of the politics of colonial war is more likely to be seen in the next few months. British colonial forces used blackmail, torture, bribery and every other resource to crush opposition, sideline it or undermine its foundations. "Counter-gangs" were British forces fighting under cover as the enemy, trying to provoke attacks and infiltrate guerrilla groups. At its worst, this boiled down to the use of government death squads.

America is not good at this kind of warfare.

The last comparable war it fought was in Vietnam. It used "search and destroy" missions, where the objective was attrition of the enemy through large-scale firepower, rather than manoeuvring around them. It failed, over and over again.

US forces are ill-prepared. Partly because of its failure in Vietnam, the US aimed subsequently to restrict itself to wars where it could win using overwhelming force. In Grenada and Panama, its forces faced traditional enemies and beat them hands down. But it intervened in Lebanon, and then withdrew abruptly in 1983 after a truck bomb destroyed the Marine base, killing 241. It tried to seize Somali leader Mohamed Farah Aideed and withdrew after losing 18 soldiers.

Because of its dread of repeating what happened in Vietnam, the US forces try to separate political ends and tactical means. They prefer to use overwhelming force, and see their key strength as technological might.

When will it all end? Victory can only be defined in political terms. There will never be a stage when the US can say that the "war" is over. But the chilling prospect is for a conflict that goes on until one side or the other runs out of the will to continue; and if Somalia, Lebanon and Vietnam are any example, it may not be the terrorists who blink first.