Ankara and Washington's dirty little secrets have kept the war in Syria going

Yet the truth is that each of the many foreign powers involved in the Syrian conflict has been reluctant to intervene directly, doing so only when they felt that their worst-case scenario was in danger of being realised.

For Washington, the worst-case scenario was that Islamic State should manage to establish a de facto state in Syria and Iraq, and it carried out air strikes to prevent that happening, even as it backed off pursuing the Assad regime for its massive violations of human rights.

For Moscow, the worst-case scenario was the fall of the Assad regime in Damascus. Only when that possibility loomed - and with it the loss of Russia's platform for influencing events in the Middle East - did the Kremlin intervene directly.

For Ankara, the Assad regime's fall was desirable - provided someone else did the actual fighting. But Bashar al-Assad's survival, while troubling, did not rise to the level of a worst-case scenario. As Turkish President Recep Tayyip Erdogan has shown with his Russian counterpart Vladimir Putin and Israeli Prime Minister Benjamin Netanyahu, even the most damaged diplomatic relationship is not beyond repair.

For Ankara, the worst-case scenario began to take shape in early August, when a US-backed, Kurdish-led force managed to liberate the Syrian town of Manbij from Islamic State. The nightmare of the Turkish government and military has always been that Syria's entire northern border would be held by Kurdish militia.

Those who say that Turkey has been waiting for an excuse to intervene are deluded. When a tiny sliver of Turkish heritage in the north of Syria came under threat, Turkey had an excuse. Instead it stole into Syria, rescued the remains of its national hero and stole back across the border before the Syrian regime could even email a complaint to the UN.

Before that there was the humanitarian crisis at Kobane, where a town full of Kurds was overrun by IS as the world stood by. Fairfax correspondent Ruth Pollard travelled to that frontier and watched as Turkish troops, less than a kilometre from the fighting, beat and gassed Kurdish protesters on their side of the border, their backs turned to Syria.

But the idea of a contiguous Kurdish proto-state sitting right next to Turkey's restive south-east - itself a hotbed of Kurdish separatism - was too much to bear. Finally a finger of the Turkish military machine was placed in the jaws of the conflict, hoping to keep Kurdish troops from advancing west of the Euphrates River.

Turkey campaigned for a long time to have a "safe zone" created in Syria's north, by which it meant a place that was off-limits to Assad's air strikes and Kurdish forces, to no avail. So now it has effectively taken the task upon itself. But for how long? And with what aim?

If there is not a wider ceasefire, there is no way that Turkey can hope to repatriate any of the millions of Syrians currently taking refuge in Turkey. What is more, if Turkish forces advance much further they will find themselves on the doorstep of Aleppo, a major Syrian city which - like Kobane before it but on a much larger scale - is in the grip of a humanitarian crisis.

Will they once again turn their back and pretend that this is nothing to do with them? If so, then Mr Erdogan's claims to be moved by the suffering of Syria's people will be exposed - and the possibility of a rapprochement with Dr Assad will move a step closer.

If the possibility of such a deal is Turkey's dirty little secret, then in Hangzhou on Monday the Pentagon's dirty little secret came close to the surface.

Russia's main sticking point at the latest meeting with US Secretary of State John Kerry was what it calls "separation of forces" - the need for the US to create "a real, genuine demarcation between terrorists and the so-called opposition", as Russian Deputy Foreign Minister Sergei Ryabkov put it.

In US media, the same distinction is made between "good rebels" and "bad rebels", with the understanding that while the Free Syrian Army are to be supported, Islamic State, the al-Qaeda-affiliated Nusra Front and other jihadist elements are beyond the pale.

But as Moscow knows only too well, on the ground this distinction is nonsense. Even within the Free Syrian Army, units such as Fursan al-Haq (the Knights of Truth), which the CIA supplied with TOW anti-tank missiles to some effect earlier in the war, use religious language and imagery in their propaganda that would send your average American voter straight to the Department of Homeland Security.

The rebellion against the Assad regime is just like the politics of the wider Middle East - it has an Islamist component that can't be excluded or wished away. And so Washington can't meet Moscow's "reasonable" request, and so the war grinds on, chewing up and spitting out the lives of millions of Syrians while everyone washes their hands, says they have done the best they could and boards the next plane to the next summit meeting.

Maher Mughrabi is the Foreign Editor of The Age and The Sydney Morning Herald.