The US may be compelled to work with Bashar al-Assad, publicly or covertly, if Isis appears on the brink of taking Aleppo from anti-government rebels.
The US has already indirectly helped the Syrian regime by passing on information about jihadi positions via the German intelligence agency the BND.
Tabqa airbase, in the Isis stronghold of Raqqa province, is under siege from jihadists. If it was to fall, Isis would have a clear run on Hama, the country's fourth-largest city.
On Thursday, the chairman of the joint chiefs of staff in Washington, General Martin Dempsey, answered his own question by asking: "Can [Isis] be defeated without addressing that part of the organisation that resides in Syria? The answer is no."
The US understands that Isis can survive forever if it has a safe-haven in Syria and that a consistent policy towards the self-styled Islamic State on both sides of the Iraqi-Syrian divide is needed.
Chas Freeman, former US ambassador to Saudi Arabia, told The Independent that General Dempsey's comments "imply the sharing of intelligence with the opponents of Isis, even those from whom we ourselves are estranged. Odder things have happened in the Middle East".
Isis might have mainly come to the world's attention due to its stunning gains in Iraq, but Syria ultimately offers the greater opportunities for expansion. Two-thirds of Syrians are Sunni Arabs - from whom Isis draws support - compared to one-fifth of Iraqis.
The official western policy in Syria has been to support 'moderate', secular rebels such as the Free Syrian Army, but it is increasingly weak and marginalised. Even other jihadi groups such as the al-Qaeda backed Jabhat al-Nusra, and Ahrar al-Sham and the Islamic Front have been unable to stop Isis.
Despite heavy losses to Isis, the Syrian army has still shown itself to be much more effective in combat against their militants than the Iraqi army, which has suffered defeat upon defeat.
Regardless of how many air-strikes the US and the west launches against Isis, they will do nothing to halt the flow of foreign fighters through the porous 550-mile Turkish-Syrian border, which Turkish prime minister and president-elect Recep Tayyip Erdogan has deliberately kept open to undermine Assad.
It is almost impossible for the west to outright back Assad, as that would mean admitting the policies of the last three years - during which 190,000 people have died - had been a complete failure.
Mr Freeman told The Independent he believes Isis cannot be defeated via a direct assault - rather isolated and left to self-destruct. But that would require the cooperation of Syria, Saudi Arabia, the other Gulf States, Iran, Russia and Turkey.
The Jihadis Return: Isis and the New Sunni Uprising' by Patrick Cockburn, published by OR Books, is available at orbooks.com