In the event, on 24 November last year a Turkish F-16 shot down a Russian bomber, killing one of the pilots, in an attack that had every sign of being a well-prepared ambush. Turkey claimed that it was responding to the Russian plane entering its airspace for 17 seconds, but the Turkish fighters made every effort to conceal themselves by flying at low altitude, and they appear to have been on a special mission to destroy the Russian aircraft.

The shooting-down – the first of a Russian plane by a Nato power since the Korean War – is important because it shows how far Turkey will go to maintain its position in the war raging on the southern side of its 550-mile border with Syria. It is a highly relevant event today because, two months further on, Turkey now faces military developments in northern Syria that pose a much more serious threat to its interests than that brief incursion into its airspace, even though Ankara made fresh claims yesterday over a new Russian violation on Friday.

The Syrian war is at a crucial stage. Over the past year the Syrian Kurds and their highly effective army, the People’s Protection Units (YPG), have taken over half of Syria’s frontier with Turkey. The main supply line for Islamic State (Isis), through the border crossing of Tal Abyad north of Raqqa, was captured by the YPG last June. Supported by intense bombardment from the US Air Force, the Kurds have been advancing in all directions, sealing off northern Syria from Turkey in the swath of territory between the Tigris and Euphrates.

The YPG only has another 60 miles to go, west of Jarabulus on the Euphrates, to close off Isis’s supply lines and those of the non-IS armed opposition, through Azzaz to Aleppo. Turkey had said that its “red line” is that there should be no YPG crossing west of the Euphrates river, though it did not react when the YPG’s Arab proxy, the Syrian Democratic Forces (SDF), seized the dam at Tishrin on the Euphrates and threatened the IS stronghold of Manbij. Syrian Kurds are now weighing whether they dare take the strategic territory north of Aleppo and link up with a Kurdish enclave at Afrin.

Developments in the next few months may determine who are the long-term winners and losers in the region for decades. President Bashar al-Assad’s forces are advancing on several fronts under a Russian air umbrella. The five-year campaign by Turkey’s President Recep Tayyip Erdogan’s to overthrow Assad in Damascus, by backing the armed opposition, looks to be close to defeat.

Turkey could respond to this by accepting a fait accompli, conceding that it would be difficult for it to send its army into northern Syria in the face of strong objections from the US and Russia. But, if the alternative is failure and humiliation, then it may do just that. Gerard Chaliand, the French expert on irregular warfare and the politics of the Middle East, speaking in Erbil last week, said that “without Erdogan as leader, I would say the Turks would not intervene  militarily [in northern Syria], but, since he is, I think they will do so”.

Erdogan has a reputation for raising the stakes as he did last year when he failed to win a parliamentary majority in the first of two elections. He took advantage of a fresh confrontation with the Turkish Kurds and the fragmentation of his opponents to win a second election in November. Direct military intervention in Syria would be risky, but Mr Challiand believes that Turkey “is capable of doing this militarily and will not be deterred by Russia”. Of course, it would not be easy. Moscow has planes in the air and anti-aircraft missiles on the ground, but Putin probably has a clear idea of the limitations on Russia’s military engagement in Syria.

Omar Sheikhmous, a veteran Syrian Kurdish leader living in Europe, says that the Syrian Kurds “should realise that the Russians and the Syrian government are not going to go to war with the Turkish army for them”. He warns that the ruling Kurdish political party, the PYD, should not exaggerate its own strength, because President Erdogan’s reaction is unpredictable.

Other Kurdish leaders believe that Turkish intervention is unlikely and that, if it was going to come, it would have happened before the Russian jet was shot down. That led to Russia reinforcing its air power in Syria and taking a much more hostile attitude towards Turkey, giving full support for Syrian Army advances in northern Latakia and around Aleppo.

For the moment, the Syrian Kurds are still deciding what they should do. They know that their quasi-state, known as Rojava, has been able to expand at explosive speed because the US needed a ground force to act in collaboration with its air campaign against Isis. Russian and American bombers have, at different times, supported the advance of the SDF towards Manbij. On the chaotic chess board of the Syrian crisis, the Kurds at this time have the same enemies as the Syrian Army, but they know that their strong position will last only as long as the war.

If there is no Turkish intervention on a significant scale then Assad and his allies are winning, because the enhanced Russian, Iranian and Lebanese Hezbollah intervention has tipped the balance in their favour. The troika of regional Sunni states – Saudi Arabia, Qatar and Turkey – have failed, so far, to overthrow Assad through backing the Syrian armed opposition.

Their enthusiasm for doing so is under strain. Saudi Arabia has a mercurial leadership, is enmeshed in a war in Yemen, and the price of oil may stay at $30 a barrel. Qatar’s actions in Syria are even more incalculable. “We can never figure out Qatar’s policies,” said one Gulf observer in frustration. A more caustic commentator, in Washington, adds that “Qatari foreign policy is a vanity project”, comparing it to Qatar’s desire to buy landmark buildings abroad or host the football World Cup at home.

In Syrian and Iraqi politics almost everybody ends up by overplaying their hand, mistaking transitory advantage for irreversible success. This was true of a great power like the US in Iraq in 2003, a monstrous power like Isis in 2014, and a small power like the Syrian Kurds in 2016. One of the reasons that Iran has, thus far, come out ahead in the struggle for this part of the Middle East is that the Iranians have moved cautiously and step by step.

Turkey is the last regional power that could reverse the trend of events in Syria by open military intervention, a development that cannot be discounted as the Syrian-Turkish border is progressively sealed off. But, barring this, the conflict has become so internationalised that only the US and Russia are capable of bringing it to an end.