ISTANBUL • Turkey has sent tanks, warplanes and special operations forces into northern Syria in its biggest plunge yet into the Syrian conflict, enabling Syrian rebels to take control of a key Islamic State in Iraq and Syria (ISIS) stronghold within hours.
The operation on Wednesday, assisted by United States air strikes, is a significant escalation of Turkey's role in the fight against ISIS - the militant extremist group ensconced in parts of Syria and Iraq that has been increasingly targeting Turkey.
By evening that day, Syrian rebels, backed by the US and Turkey, declared they had seized the town of Jarabulus and its surroundings, which had been ISIS' last major redoubt near the Turkish border. It appeared that most of the militants had fled without a fight.
The offensive had two immediate goals. First, clear ISIS militants from their remaining border stronghold, and, second, roll back recent advances by Syrian Kurdish militias that Turkey considers an equal or greater threat because of their links to its own domestic Kurdish insurgents.
Yet, it had deeper reverberations, signalling a broad and volatile reshuffling of alliances in and around Syria. And with Russia, the Syrian government's main ally, issuing only a tepid condemnation of Turkish incursion, there were signs that Moscow and Washington could be testing out taking baby steps towards a compromise.
President Barack Obama's chief spokesman Josh Earnest said the Turkish assault was "an indication of important progress" in the campaign against ISIS.
US Vice-President Joe Biden travelled to the Turkish capital Ankara on Wednesday to meet Turkish President Recep Tayyip Erdogan. Speaking after the meeting, Mr Biden said Syrian Kurdish militias, an important US ally, would have to meet a Turkish demand by withdrawing to the eastern side of the Euphrates River in north-eastern Syria.
He said: "We have made it clear to Kurdish forces that they must move back across the river. They cannot, and will not, get American support if they do not keep that commitment. Period."
It was an unusually sharp warning from the US to the Kurdish-led forces, which US officials have repeatedly called their most reliable partner on the ground against ISIS. In return, the US got something it has pushed for in vain for years - getting Turkey to take a more proactive stance to battle ISIS fighters on its border, after allowing them to cross it with impunity for years.
The solution appeared to be allowing Turkey to try to clear the area of both ISIS and Kurdish militias. Turkey has also signalled an even bigger shift in recent days - that it is prepared to take a more aggressive diplomatic role in Syria, working alongside Iran, Russia and the US to seek an end to the war.
The Turkish government has long insisted that Syrian President Bashar al-Assad would have to step down before peace talks could be held. But lately, Turkey has softened its stance, indicating that it would accept a role for him during a political transition.
The Syrian Foreign Ministry condemned the operation as a breach of Syria's sovereignty.
Russia, in a notably softer reaction, said it was "deeply concerned". It did not refer to the rebels as "terrorists", a label it has applied to all opposition groups in the past. This time, it called them "opposition fighters".
The rebels involved in the operation appeared to be mainly from the groups fighting to unseat Mr Assad. The US, Turkey and other allies support them through a covert operations centre in Turkey, and they identify themselves as part of the Free Syrian Army.
The Free Syrian Army is more a brand than a command structure, led by army defectors and others who say they reject the extremist ideologies of Al-Qaeda and ISIS.
The efficiency of Turkish military operations also marked a major boost for the Turkish army, whose reputation was tarnished by the failed July 15 coup against Mr Erdogan. Rogue elements in the armed forces were among those involved in the coup.
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