Crucially, this latest initiative involves three of the four big local players – Saudi Arabia, Turkey and Egypt – as well as some smaller local actors like the UAE, Qatar and Bahrain. For once, regional states are taking a lead in trying to restore order in their own neighbourhood.

“The war against Isil is only the tip of a much bigger Middle East war unfolding before our eyes”

All this is tardy but nevertheless welcome, and it may yet rescue the Government’s open-ended air campaign against Isil from drifting into irrelevance. There is much that the US and the UK can add to the new coalition’s command-and-control centre to be established in Riyadh. A coordinated air/ground campaign is now a genuine prospect. Indeed, Isil fighters know that ground forces are coming for them in Mosul, Palmyra and Raqqa, because they are making elaborate preparations. When they do come, the resulting battles will be desperate and hard on public sensibilities.

This newfound military impetus should also add real urgency to the political processes in Vienna and New York – currently an exercise in diplomatic form over substance. The Western-orientated coalition now has a substantial bargaining chip to set against the forces of Assad, the Russians, and Iranian-led militias that have been in poll position since the summer.

But herein lies the problem. The war against Isil is only the tip of a much bigger Middle East war unfolding before our eyes. It pits the Muslim world against itself – Sunni versus Shia; Saudi versus Iran. And with the West supporting the Saudi-led Sunnis, and Russia the Iranian-led Shia, it threatens a dangerous renewal of Cold War-style conflict by proxy.

The signs of this are ominous. In theological terms, Saudi Wahhabism lies at the heart of the Sunni perversion of Islam that forms Isil. Yet the Saudis have long since lost control of the extremists. Indeed, Isil’s attraction for many ordinary Sunnis is popular frustration at how badly their autocratic and corrupt governments have let them down over the years, and how frightened they now are at a Shia resurgence across the Middle East that began with the Iranian revolution in 1979.

Britain has no direct interest in being part of a regional, sectarian war. The West simply wants to snuff out Isil, while adopting a strictly diplomatic approach to the broader regional problem. But sticking to our war against Isil without becoming part of the larger proxy war now taking shape will be a considerable achievement. What if the Saudi-led forces face defeat at the gates of Isil’s de facto capital in Raqqa? Would coalition air support alone save them? Or would some greater ground commitment become imperative?

For Western powers, the Saudi initiative promises to help achieve our main and only military objective – the defeat of Isil. But for the Saudis themselves, defeating Isil is only the first step in wresting back control over the Sunni communities of the Levant and Arabia. They could be at the beginning of a long war. How far will we go along with them?

There are disturbing historic echoes in the two great struggles between Sunni and Shia Muslims – in the Seventh and 16th centuries. In a region where grudges, animosities and spilt blood are particularly long-remembered, the repercussions are still felt today.

“The fact that Washington and London seem to know so little about this Saudi initiative is not a good sign”

Of course, the presence of the US, Russia, Britain, France and Germany in this regional equation might have a sobering effect on all participants and help keep the military objectives limited. But the fact that Washington and London seem to know so little about this Saudi initiative is not a good sign; they appear to be trailing in the wake of discreet Saudi diplomacy and are not yet in a position to react in more than general ways – let alone direct matters.