Three years ago, when the world obsessed about President Assad, some of us warned that Syria was only one frontline in a wider sectarian war between Sunni and Shia; that the spread of militant jihadism among the Sunni community, funded by Saudi Arabia and Qatar, was a preparation for this. And that before long this movement, like the 30 years’ religious war of 17th-century Europe, would threaten to engulf the entire Muslim world.

This is the true context in which the Isis terror in the Middle East must be seen. It is why we need to understand that, though the world watches Iraq today, just as it did Syria yesterday, the actual war being fought is a regional one, with potential to spread across Islam worldwide. It is not an accident that many Isis fighters are foreigners – many of them not even Arabs. Or that they use the most modern global communications to evangelise their medieval horrors.

Of course, seeing the humanitarian crisis in northern Iraq – for which the UN has just declared its highest state of emergency – something must be done. But then we said the same about the slaughter in the now forgotten suburbs of Damascus. What we need now is not just a plan for a tragedy, but a strategy for a widening war.

What is happening in the Middle East, like it or not, is the wholesale rewriting of the Sykes-Picot borders of 1916, in favour of an Arab world whose shapes will be arbitrated more by religious dividing lines than the old imperial conveniences of 100 years ago.

For as long as western policymakers deny, even tacitly, that this is the most likely outcome of present events, so they will fail to find solutions to the Middle Eastern conundrums that confront us. And so we come to the case of the beleaguered Kurds, and the Yazidis on Mount Sinjar. And so, we drop humanitarian aid.

But then what? We did the same in Srebrenica in 1995. It worked well enough for a few days. But in the absence of a credible western policy in Bosnia, it only gave space for mass murder later. So what credible policies are available to us in Iraq?

There are three. The first is an all-out, long-term western military engagement to defeat Isis and save Baghdad. This is favoured by some who have not yet learned the lessons of Iraq and Afghanistan, and a few superannuated generals seeking more spending on defence. It is by far the least practical and most unwise option open to us. Western populations would not support it, and we no longer have the military means to do it.

The second is to help the Iraqi state to defeat Isis itself. This seems to be current western policy. But I fear it amounts to little more than elevating desperate hope over reasonable expectation. It was the collapse of the Iraqi army that gave Isis the advanced American weapons they now use to drive back the Kurds. And it has been the subsequent absence of any effective government in Baghdad that has allowed the jihadists to continue widening their advance on all fronts.

The Potemkin reconstruction of the Iraqi government in the last few days is unlikely to alter a fundamental truth: the Iraqi state is not, and is unlikely to become, an effective instrument for a western-backed attempt to tackle the Isis insurrection. Unless of course Iran too gets directly involved. But that would inevitably lead to the creation of a de facto greater Iran extending into Iraq, and to a further widening of the sectarian faultlines. This may not be avoidable – but should we be encouraging it?

The third option is to help the Kurds by all means possible – assistance to house the Yazidis, equipment, military training, advice, protective air strikes – anything short of current operational boots on the ground. The aim would be to make Iraqi Kurdistan the northern bulwark against the Isis advance. The government seems at last to be tiptoeing in this direction – but why so half-hearted? It’s a strange scruple that flies in other people’s weapons but denies access to our own. Is there a difference?

But there are downsides here too – big ones. Whether intentionally or not, we will end up acting as handmaiden to Kurdish ambitions for full independence – and in so doing, effectively assist in the dismemberment of Iraq. Part of the deal with the Kurds would have to be an end to interference in Turkey, which has its own problems with Kurdish secessionism. We would also be tacitly accepting the end of the Sykes–Picot borders in the Middle East.

So this will only work if it is not just a short-term plan, but part of an integrated long-term strategy: a new rapprochement with Iran to act as a counter-balance to those who promote Sunni jihadism; deeper engagement with Turkey; greater pressure on those Gulf states that fund jihad (is the government’s reluctance here because of Tory friends among the Gulf states?); and a new determination to deal with illegal Israeli settlements, as a prelude to a lasting peace in Palestine.

None of this will be easy, of course. But better, surely, to face up to the realities of the post-Sykes-Picot Middle East and influence it where we can, than lose the moment standing impotently by, hoping that yesterday will come back again.