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US president Donald Trump's decision to withdraw 2,200 troops from Syria is a destabilising game-changer. He declared victory over Islamic State in Syria on Wednesday even though the group has 14,000 fighters in the country and remains a threat to the region.
Islamic State, also known as Isis, has reverted to guerrilla tactics and continues to clash with US-allied Kurdish forces and the Syrian army. While France and Britain remain committed to military operations in Syria, Arab rulers have ignored the suggestion they could replace US forces in the coming, unpredictable phase of the Syrian war.
Isis will step up attacks if the US abandons Kurdish allies who have battled the jihadis over the past three years and established a self-declared autonomous area in north and east Syria, which makes up 25 per cent of the country.
Turkish president Recep Tayyip Erdogan vowed to tackle Isis after Trump gave a "positive response" to his plan to launch a military operation against the US-trained and armed Kurdish People's Protection Units (YPG) in a band of territory 15-20km wide on the Syrian side of the border.
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Erdogan regards the YPG – the backbone of the US-led campaign to defeat Isis in Syria – as an offshoot of the Turkish Kurdish Workers' Party, which has rebelled against Ankara for 30 years.
Turkey invaded and occupied the northwestern Syrian Afrin district early his year and drove out YPG fighters and half of the 230,000 mainly Kurdish inhabitants. The US did not respond.
If the US withdrawal goes ahead and fails to deter Turkey or provide aerial protection for the Kurds, Trump would leave the YPG to face a vastly more powerful Turkey armed with US heavy weaponry and US war planes. Syria's allies, Russia and Iran, would not intervene.
Russia seeks to split Turkey from US and Nato alliances while Iran is determined to retain Turkey as a customer for its oil and gas, as well as a trading partner.
Although the Kurds have been negotiating a return to rule by Damascus, the undermanned and overstretched Syrian army is in no position to defend them. If a Turkish invasion is averted, the Kurds would have to renounce their demand for autonomy in a federal state and accept Damascus's terms and central control. This could calm Ankara.
Northern Syria
Moscow and Iran have welcomed US withdrawal even though it could detonate the explosive situation in northern Syria. Turkey could not only occupy the border zone but also consolidate its grip on insurgent-controlled northwestern Idlib province. Ankara is poised to take over Idlib as the Turkish military has set up posts in and around the province to monitor a ceasefire imposed by Russia, Turkey and Iran.
Trump’s pull-out would leave the US without leverage in the process of fashioning a post-war political deal for Syria’s governance, making Russia the major power-broker. President Bashar al-Assad’s survival has been assured and the Saudi-sponsored opposition has been sidelined since the Syrian army regained 65 per cent of Syrian territory.
Russia's role in the region and on the international stage has been boosted. Aware that the US has lost its appetite for regional military intervention, Turkey, Iran and several Arab states have been courting Russia. The political clout of Saudi Arabia, the Trump administration's main Arab partner and Iran's rival, has diminished.
Iran could wield considerable influence in Syria and Lebanon, prompting Israel to increase attacks on Iranian forces in Syria and risking Russian and Iranian responses. Tehran dismisses Israel's demands for Iran's withdrawal from Syria and an end to Iran's support for Hizbullah, which is regarded by Israel as a major threat.
A miss-step by any party could provoke fresh conflict.