Syria is the most challenging theatre for Barack Obama's plan to degrade and destroy the jihadis of Islamic State (Isis). After three-and-a-half years of seesawing over the bloodiest conflict of the Arab spring, the US president's new strategy constitutes a potentially significant intervention – but one whose consequences are extremely hard to predict.

Air strikes without crossing the Rubicon of American "boots on the ground" means ramping up currently limited western and Arab support for rebels fighting to overthrow Bashar al-Assad. Saudi Arabia's agreement to host training for armed groups widens Obama's "coalition of the willing", but it also serves as an ironic reminder that reckless Gulf backing for Islamist brigades helped create the Isis monster now dominating the wider Syrian tragedy.

Action in Iraq faces its own myriad of problems but it is easier in the sense that it comes at the request of the Baghdad government and the Kurds, and with their active cooperation. Syria is different.

It came as no surprise that Obama made clear there was no question of working with Assad, who believed he would come to be seen as a lesser evil than the jihadis he had always falsely blamed for the uprising and become an indispensable partner in a new "war on terror".

He has probably not given up, suggests Jihad Makdissi, a former Syrian diplomat. He says Assad will probably still volunteer to share intelligence to target Isis – while ritualistically condemning attacks on his soil. It is a reasonable bet that Syria's air defences will not be in action too vigorously.

The US shift was quickly welcomed by the western-backed Syrian National Coalition and Free Syrian Army (FSA). But they want a fight to overthrow Assad's "tyranny" – which is not quite the declared goal of Washington's Isis strategy. Still, just over a year after Obama failed to enforce his own "red line" over Assad's use of chemical weapons, the often hapless opposition now sees a fresh chance for far more effective backing.

Saudi and Qatari cash was never matched by the kind of coordination, leadership and heft the US alone can manage. Yet the extent of Obama's financial commitment – depending on congressional funding – is still unclear. The security challenge is daunting too: expanding a limited CIA-supervised effort backing vetted FSA units will heighten alarm about US weapons falling into the "wrong hands" – as they did so spectacularly in Iraq when Mosul fell in June.

But Salafi-jihadi fighters are the ones who dominate the Syrian battlefield and are the most capable of defeating Isis: their goal also remains overthrowing Assad. In one early reaction to Obama's speech, FSA and Kurdish units in the Aleppo area announced the creation of a new joint operations centre – Euphrates volcano.

Obama has defined his mission in Syria as defeating the jihadis without helping Assad – or at least without helping him too much. Yet some warn of an opposite effect: that beefed-up US (and wider) backing for the rebels could tip the balance of war against Damascus, its armed forces ground down. An associated risk is the marriage of convenience between the US and Iran – the latter staunchly backing Assad while helping fight Isis in Iraq. This is all about possibilities, not certainties. The role of Russia, another loyal ally of the Syrian government, is also unclear.

Isis gains in Iraq – made possible because of their havens across the now erased border – have meant that Syria's war has faded from public view, at least in the west, where the beheading of two US journalists made an impact not achieved by the deaths of 200,000 Syrians.

Barely visible efforts to end the conflict have continued this week with the arrival in Damascus of a new UN envoy – Staffan de Mistura – picking up where his two predecessors failed. "The international community and the UN must help the Syrians find a political solution to the crisis," de Mistura said in a statement on Thursday. Isis is now firmly in US sights. But making peace in Syria looks even harder than before.