Turkey is back in the game in Syria. Turkish tanks and special forces last week joined anti-Assad rebels to capture the border town of Jarabulus from Isis. It was the first significant Turkish ground operation in Syria since the beginning of the war in 2011. The immediate target was Isis, but a more important Turkish objective is to strike at the political and military power of the Syrian Kurds who are already in control of much of the territory south of the Syrian-Turkish frontier.

The war in Syria has reached a critical moment which may see it de-escalate or explode into even greater violence. US Secretary of State John Kerry and the Russian Foreign Minister Sergei Lavrov met on Friday in Geneva with the aim of reaching a political and military cooperation agreement. Asked to identify the main obstacle to a ceasefire, Mr Lavrov said: “I don’t want to spoil the atmosphere for the negotiations.” Moscow has already proposed a 48-hour ceasefire in Aleppo.

It is as if all side are consolidating their positions in expectation of a shift towards more war or greater peace. As Mr Kerry and Mr Lavrov met in Geneva, ambulances, buses and trucks were assembling outside the rebel-held suburb of Daraya in Damascus which government forces besieged for four years. Under an agreement reached on Thursday, some 4,000 anti-Assad fighters and local people are to be evacuated to rebel-held areas.

But government control is not extending everywhere in Syria as in the north eastern provincial capital of Hasakah Syrian Kurdish fighters have driven out the Syrian Army and its local paramilitary allies from most of the city.

The biggest change in the political and military landscape is the Turkish intervention, though its extent remains unclear. The aim may be primarily defensive, limited to controlling the zone between Jarabulus and the Kurdish enclave of Afrin 70 miles to the west through Arab and Turkoman proxies backed by the Turkish armed forces. Alternatively, Turkey could build up an anti-Assad and anti-Kurdish base area north of Aleppo making it a main player in the region. The first purpose is already largely achieved with apparent US support, Russian tolerance and muted criticism from the Damascus government, suggesting that Turkish action was not unexpected by all three.

Broader Turkish involvement, though tempting to some in Ankara, would embroil Turkey in the lethal swamp lands of the Syrian-Iraqi war. Turkey may be able to prevent the Kurds permanently extending their rule west of the Euphrates, but it would be a very different and more dangerous operation to attack the de facto Syrian Kurdish state, which has spread itself between the Euphrates and the Tigris rivers since the Syrian Army largely withdrew from the region in 2012. At the same time, so long as this Kurdish quasi-state exists. the Turkish state is endangered.

Turkish-backed rebels secure Jarablus in northeastern Syria

It is an iron rule of politics in the Middle East that everybody at some point overplays their hand. The Israelis did so when they invaded and tried to dominate Lebanon in 1982 and the Americans did the same when they overthrew Saddam Hussein in 2003 and sought to become the predominant power in Iraq. The Syrian Kurds used the military effectiveness of their 50,000 fighters (these are there numbers which may be exaggerated) and the massive destructive firepower provided by the US air force, to overrun much of north east of Syria over the last eighteen months. Buoyed up by their victory over IS in the four-and-a-half siege of Kobani, which ended in early 2015, the ruling Democratic Union Party (PYD), the Syrian arm of the Kurdistan Workers Party (PKK) in Turkey - and their formidable paramilitary forces, the People’s Protection Units (YPG) – have repeatedly defeated Isis.

But they too may have gone a step too far, advancing beyond the point that they could rely on the support of their US and Russian allies to stop Turkey intervening. The capture of the town of Manbij from Isis on 12 August by the Syrian Democratic Forces (SDF), a grouping dominated by the YPG, led to a further move towards Jarabulus to the north and Isis-held al-Bab to the east. This finally precipitated Turkish military intervention which had been under consideration for over a year. Turkey was faced with disaster in Syria, where it had failed to depose President Bashar al-Assad and opened the door to a section of the PKK, at war with the Turkish state for 32 years, setting up its own well-armed state. With a sanctuary in northern Syria, the PKK could sustain a guerrilla war in Turkey for as long as it wanted (much as the Taliban is undefeatable in Afghanistan because of its sanctuaries in Pakistan). IS, to whose activities Turkish intelligence had once turned a blind eye when it did not provide covert assistance, was carrying out repeated terrorist bomb attacks in Turkey.

In pictures: Turkey launches operation in Syria Show all 9 1 /9 In pictures: Turkey launches operation in Syria In pictures: Turkey launches operation in Syria Turkish tanks driving to the Syrian-Turkish border town of Jarabulus yesterday AFP/Getty In pictures: Turkey launches operation in Syria Turkish-backed gather on the outskirts of Jarabulus, Syria, ahead of an offensive on 24 August 2016 Reuters In pictures: Turkey launches operation in Syria Turkish army tanks make their way towards the Syrian border town of Jarabulus, Syria August 24, 2016 Reuters In pictures: Turkey launches operation in Syria Turkish soldiers return from Syria to Turkey with tanks after a military operation at the Syrian border as part of their offensive against the Islamic State (IS) militant group in Syria, Karkamis district of Gaziantep, Turkey, 25 August 2016 EPA In pictures: Turkey launches operation in Syria Turkish army tanks and Turkey-backed Syrian opposition forces move toward the Syrian border as pictured from Karkamis, Turkey, AP In pictures: Turkey launches operation in Syria Turkish tanks on their way to the Turkish-Syria border during an operation against Isis on 24 August 2016 EPA In pictures: Turkey launches operation in Syria Syrian opposition fighters being transported during preparations to enter Jarabulus in Karkamis, Turkey, on 24 August 2016. EPA In pictures: Turkey launches operation in Syria An air strike hitting Isis-controlled territory near Jarabulus, near the Turkish border, on 24 August 2016. EPA In pictures: Turkey launches operation in Syria A Turkish army tank and an armoured vehicle stationed near the border with Syria. Turkish media reports say Turkish artillery has launched new strikes at Isis targets across the border AP

A Syrian Kurdish state-let fuelling a guerrilla war inside Turkey is already a reality and Turkish military intervention has long been on the cards. Delaying it was the YPG’s position as the most powerful US ally on the ground in Syria. US-Turkish relations deteriorated because of Turkey’s failure to seal the Syrian-Turkish border against Isis and other Jihadis from the north, so the US was happy to see the YPG do so from the south on the Syrian side of the frontier. The US did not want the YPG diverted from fighting Isis by Turkish action while Russia, one of whose aircraft was shot down by Turkish planes on 24 November 2015, was determinedly against Turkish military intervention.

The multi-sided war in Syria is becoming even more complex than before. The Syrian and Iraqi Kurds have made great advances on the ground that they could not have carried out without international - and primarily American - support. The YPG is today fighting in areas that have mixed Kurdish and Arab populations or where the Kurds are a minority. The Iraqi Kurds used the opportunity presented by the IS capture of Mosul in 2014, to seize territory long disputed with the Iraqi government in Baghdad. Kurdish leaders know there will be a day of reckoning and have long pondered how to keep their gains once their foreign allies no longer feel threatened by Isis and need their support.

All factions in Syria are to a greater or lesser degree the proxies of foreign powers and cannot go on fighting without their backing. These parties, regardless of whether they are pro or anti-Assad, Shia or Sunni, Arab or Kurdish, try to manipulate their foreign allies, suspecting that they will one day betray them in order to serve their own interests. But Syrian contenders cannot do without these foreign allies: President Assad would like to fight on until victory but must have the backing of Russia, Iran and the Shia in Iraq and Lebanon. The anti-Assad Jihadis dominate the armed opposition in Syria, but cannot fight without backing from Turkey, Saudi Arabia and Qatar.