Everyone opposing President Bashar al-Assad of Syria will be invited to Riyadh later this month with one significant exception: a delegation from the so-called “Islamic State”.

At least 65 “opposition figures”, in the words of Saudi Arabia’s state-controlled press, are supposed to achieve the impossible – Arab unity – in time for the new year’s round of multinational peace talks on Syria. But the whole shebang is likely to prove as mystifying as David Cameron’s 70,000 “moderate” fighters. There will, we are assured, be representatives of the “armed opposition”. But who are they? Will the head-chopping and sectarian al-Qaeda outfit Jabhat al-Nusra be represented, funded by sources in Qatar and posing as the new “moderates”? And then there’s the virtually non-existent “Free Syrian Army”, which will certainly be ready to fly to Riyadh, if only to prove it exists.

Will the Kurds be there? The Turks, who are spending more time bombing them than any other groups in Syria, will not approve. The Iranians have already expressed their anger, sneering that the Saudi conference will cause the failure of the international talks in Vienna. The US Secretary of State John Kerry has, of course, given his approval – why should Washington oppose an initiative by its “moderate” Arab ally, Saudi Arabia? But then, as British MPs now know all too well, it all depends what you mean by “moderate”.

And the poor old Germans, who are now committing 1,200 soldiers, a frigate and reconnaissance aeroplanes to the Syrian war – in a strictly non-combat role, of course – were huffing and puffing yesterday that Saudi Arabia was “a key partner in regional conflict resolution”. A necessary if dodgy assertion, after the German foreign intelligence service (the BND) dumped on the Saudi Defence Minister, Deputy Crown Prince Mohammed bin Salman, for his bloody intervention in the Yemeni war.

“An impulsive policy of intervention,” was how the BND characterised the young prince’s bombing of the Shia Houthi rebels, suggesting that the prince and his father – the new King Salman – wanted to present themselves as “leaders of the Arab world”. The job of the German intelligence agency, a foreign ministry spokesman announced, was to provide “information that the government requests” and “not to supply journalists with information”. All of which suggests that the BND’s assessment of Saudi Arabia was perfectly accurate – merely handed to the wrong group of Germans.

And so we come to Isis. Since their Wahhabi tradition of sectarianism and brutality is the very same Wahhabi faith which lies at the foundation of Saudi Arabia’s version of the Sunni religion, and since much of the cult’s funding has come from Saudi Arabia, we have to ask who will represent their unique, purist and violent point of view at the Riyadh conference?

For surely they are fiercer opponents of the Assad regime than any of the opposition groups which will turn up in Riyadh. Their propensity for chopping off heads scarcely prohibits them from attending, not least because Nusra, which will assuredly have someone there, also has a habit of throat-slitting.