The purpose of war is to give yourself a strong position at a peace conference. In the days when warfare was state versus state, that is what generals used to remind politicians: we can bring the enemy to its knees, but it is you who has to design the peace. Sometime between 1991 and 2003, the US forgot this principle and the result is the situation we have now.

Iraq is effectively dismembered into Shi’a, Kurdish and Isis-run territories. Parts of Afghanistan are being reconquered by the Taliban. Syria’s disintegration has propelled millions of refugees into Europe, Turkey and Lebanon. And the Islamic-fascist regime of Isis has staged the first sophisticated “marauding attack” on European soil, killing people who had assumed, like the rest of us, that the disintegration of the Middle East was somebody else’s problem.

Now it is Nato’s problem. In British security circles, there is tacit acceptance that, if it wanted to, France could invoke Article 5 of the North Atlantic treaty, which requires Britain to render military support to an ally under attack. Article 5 is the political deterrent designed into Nato at conception. It gives all signatories the right to wage war legally, as an act of self defence, under principles recognised by the UN charter. It has only once been invoked: by the US after 9/11.

Obviously, there are legal difficulties: Isis is not recognised as a state, though it claims to be one and controls a defined geographic area. Nevertheless, the Paris attack clearly breached “the security of the North Atlantic area” – the phrase used in Article 5 to define the objective of any retaliation. If the French requested it, and major states refused, it would mark the end of the alliance’s credibility. Either way, British military planners will be right now exploring options for what to do if France requests military aid.

Nato’s modus operandi is spelled out in its “strategic concept” document, the last version of which was drawn up in 2010. Russia’s invasion and conquest of Crimea and Donetsk had already rendered it obsolete. The Russian deployment of troops and aircraft into Syria likewise. But the Isis attack on Paris demands its urgent revision.

In Nato-speak, “mobile and deployable” forces means soldiers you send to a foreign country to attack the enemy. From now on, all western democracies will have to maintain and expand such forces to be deployable at home. France was lucky in having a long-standing paramilitary police force, alongside anti-terrorist military units who, since the attack on Charlie Hebdo, had been training for a Mumbai-style event. Other European countries would have been less well-equipped than Paris to deal with what happened.

But the scale of the military redesign is nothing compared with the political rethink needed after the Paris attack. If you’ve watched social media since Friday night you will have seen wave after wave of arguments in favour of avoiding a fight with Isis. It was a “false flag operation”, allege some; Isis exists because the west armed Sunni jihadis, say others.

Next come the doomsayers: those who argue that Isis is so deeply rooted into jihadi-sympathising Muslim communities in western Europe that it cannot be fought – militarily or through intelligence and policing – without provoking a civil war.

Next comes – with greater justification – the argument about cause and effect. Isis rules Iraq because the west invaded it, smashed the civil power and imposed a new one that then failed. It rules half of Syria because the west applauded an uprising against Assad but failed to back it, while Turkey and Saudi Arabia nurtured the Islamic- fascist group. It has a foothold in Libya because of our half-assed intervention there. And across the region, it draws in recruits because of social dislocation and poverty.

You could admit all these things to be true – from the wild conspiracy theories to the sociology – and yet miss the point. Isis attacked civilians irrespective of their position on Islam or imperialist war; it attacked, specifically, symbols of a secular, liberal lifestyle. It did these things because that is what it is fighting: the west, its people, their values and their lifestyle.

In formulating the UK’s response – with or without Nato – the problems are large. The electorate mistrusts offensive military action. It fears – rightly on the basis of the evidence from Iraq and Afghanistan – that expeditionary warfare creates mainly chaos, opening a space for sectarian conflict, jihadism and killing civilians. Western electorates have no taste for the kind of allies we would need to reimpose the old “order” on the territories Isis operates in. Bashar al-Assad and Recep Tayyip Erdoğan are not only serial human-rights violators; they have each proved ready to attack those fighting Isis – the Kurds and the secular resistance.

But the biggest challenge comes if you imagine what victory would look like. Isis-held territory being reoccupied by armies that, this time, can withstand the suicide bombings, truck bombs and kidnappings that a defeated Isis would unleash. Mosques and madrassas across the region stripped of their jihadi preachers. A massive programme of economic development focused on human capital – education, healthcare and institution building – as well as physical reconstruction. Nonsectarian, democratic states in Iraq and Syria and an independent Kurdistan state spanning parts of both countries. To achieve this you would need to unleash surveillance, policing and military action on a scale that could only be acceptable to western electorates if carried out with a restraint and accountability not shown in Afghanistan and Iraq.

The alternative is to disengage, contain Isis, deal with the refugees and try to ignore the beheading videos.

In reality, this question is only really posed for three countries that have the power diplomatically and militarily to take significant action: Britain, France and the US.

But that’s not the main question Isis posed last Friday. The main question is the one John Maynard Keynes threw at Britain’s political leadership in 1939: what is the world going to look like when we win?

By answering this, the British and American populations were persuaded to endure total war in the fight against Nazism. So the question now is not how many bombs we want to drop on the HQ of Abu Bakr al-Baghdadi. It is: what do we want at the peace conference, and what will our own society look like after the struggle is over?

• Paul Mason is economics editor of Channel 4 News. @paulmasonnews