For two weeks an international coalition led by the United States has been assembling to fight Islamic State (Isis). Several European countries as well as Gulf states have joined the effort. The proclaimed goal is to damage and ultimately defeat an entity that has claimed great swaths of Iraq and Syria, declaring itself both a state and the modern incarnation of the caliphate. It has demonstrated its capacity for the most extreme forms of violence. Isis is both barbaric and highly disruptive to whatever is left of the regional order, in an already explosive part of the world.

It is, as this newspaper has already argued, painfully difficult to see how Isis’s momentum can be slowed without military action. But the question of what action, and with what endgame in mind, remains open. So far the western and Arab-aided military effort seems to have achieved relatively little. It may be too soon to draw conclusions, but the dramatic events unfolding in the Kurdish town of Kobani have exposed the limits of air power and its effectiveness. US-led forces were said to be intensifying their operations on Monday, in an attempt to prevent the complete capture of Kobani by Isis, which had already raised its black flag over the town, right on the border between Syria and Turkey. In other parts of Syria, as in Iraq, Isis remains far from defeated.

Under these circumstances it might be tempting to resort to desperate measures. Among them would be the recasting of the Assad regime as a necessary, if ugly, ally in a difficult fight. Such a move would suit the Syrian dictator well: he has long sought precisely that re-legitimisation. His work toward that goal began just as soon as Barack Obama, along with the British, French and German leaders, solemnly declared that Assad had to leave power, back in August 2011. In that season when Arab strongmen seemed to be toppling in rapid succession, he moved fast to portray all who opposed him as extremists and violent jihadists. The implicit message to the west was that they should stick with the devil they knew. It’s possible he was even content to pump up the Isis threat: he did not deploy his air force against Islamic State targets and boosted Isis ranks when he released a large number of Islamist radicals from his prisons.

Now, three years on, Damascus is confident enough to talk about a “common fight” with the west against Isis. Assad has stayed quiet while the coalition bombs targets in Syria, happy to acquiesce in what he sees as tacit validation. He might even interpret western action against one of his enemies as a green light to continue waging war in a way that produces the mass suffering, flight and death of civilians. He has deployed the full arsenal at his disposal: tanks, artillery, aircraft, as well as Scud missiles and – at an earlier stage – chemical weapons.

An estimated 200,000 Syrians have been killed in the civil war, and the repression is ongoing. Now the crisis has come to a horrible pass: as the international coalition strikes at Isis, Assad’s planes have been dropping bombs on the inhabitants of Aleppo, a city that has been a stronghold of the opposition. Whatever covert coordination is happening between Damascus and the US military against Isis, it would be a strategic error to let Assad continue this slaughter with impunity, to let him believe he has become an ally, one whose crimes will be met with silence from western capitals.

There can be no alliance with Assad, even unspoken. Not just because this would be morally questionable, to put it at its mildest, but because it would be counterproductive. Allowing Assad to become what the Americans used to call “our sonofabitch” would only increase the appeal of Isis to mainstream Sunni opinion in Syria and beyond, boosting the very forces the international coalition is set on destroying. It would be all but impossible to convince moderate Sunnis of the legitimacy of western policy if this war were to be run, in any way, in coalition with Assad’s military. The opposition Free Syrian Army has doubts enough about this intervention already.

If fighting Isis is, in part, about counter-terrorism and defending western countries against violent jihadism, then another risk should be factored in: appearing to be siding with the Assad regime will only lend more weight to the twisted narrative of Isis recruiters. Bashar al-Assad’s tactics remain a major cause of the current torment in the Middle East. He is not, and cannot be, part of the solution. Western countries should say as much.

The Guardian editorial