The latest upsurge in airstrikes, hospital bombings and civilian deaths in the rebel-held Idlib province in northern Syria has elicited little in the way of protest or pushback from the Trump administration. Its silence contrasts sharply with last week’s furious US condemnation of Iran over its handling of sporadic domestic unrest.

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The comparison is instructive. In Syria’s civil war, the US has largely ceded a free hand to Russia’s president, Vladimir Putin. And Donald Trump seems to have abandoned attempts to force Bashar al-Assad, Syria’s dictator and Putin’s ally, to step down. Trump’s principal focus is not Damascus, it’s Tehran.

The result is something approaching impunity for killers. To take one example, 11 medical facilities in Idlib, Hama and eastern Ghouta were targeted by Russian and Syrian government airstrikes and shelling over the new year period, according to the Union of Medical Care and Relief Organisations.

But Trump’s attention is elsewhere. His passionate, outspoken hostility to Iran jars badly with his passivity and disinterest over the plight of Syrians dying and injured in the war. His controversial efforts to advance ties with Putin, and his wooing of the Saudis and Israelis, Iran’s sworn enemies, fit his top priority of isolating Tehran. The Syrian child victims Trump vowed to defend last April have been forgotten.

Putin has adopted a pragmatic approach to Iran, looking for areas of common interest. But his calculations of relative advantage may be changing. The two countries collaborate on Syria, where Iran has deployed ground forces. But they have a long history of enmity. Moscow backed western-inspired curbs on Iran’s nuclear programmes. Putin’s outreach to Iran’s foes in Riyadh and Tel Aviv has induced fears in Tehran of a stab in the back.

Unlike Iran’s hardliners and the Revolutionary Guards, Putin has indicated he could accept Assad’s dethronement if a pro-Moscow regime remains in place and Russia’s strategic Middle East interests, including military bases in Syria, are respected.

But for Shia Iran, the survival of Assad’s Alawite ascendancy is key to its aim to extend its physical reach to the Mediterranean coast, control Lebanon, hem in the Saudis and put pressure on Israel.

Trump’s strong support for Israel, including his recent recognition of Jerusalem as its capital, is mirrored by Putin. “Under Putin’s leadership, Russia has developed the best relationship it has ever had with Israel,” said Josh Cohen, a US analyst.

Putin has reportedly instructed both Assad and Hezbollah not to retaliate against Israeli strikes in Syria – the latest took place on Tuesday. Israeli media claim Putin even proposed a deal preventing foreign powers from using Syria as a base for attacking a neighbouring state, a concept in conflict with Iran’s current posture.

All these developments underline how Putin has become the go-to man in the Middle East – and why Iran has much to fear from deepening Trump-Putin collaboration. Trump is threatening new sanctions this week over Tehran’s crackdown on street protests. Under US law, Trump has until Friday to decide whether to pull out of the 2015 nuclear deal. He has labelled Iran a “terrorist nation”.

It would thus be logical for Trump to seek Russia’s backing in his war of attrition with Iran. He faces two main obstacles. One is Russian pragmatism. Putin might be tempted by American recognition of Russia’s leading role in the Middle East (and a continuing free hand in Syria). But concern about the region-wide havoc the unpredictable Trump might wreak will give him pause.

The other big obstacle is the searing political toxicity surrounding all White House dealings with Russia, as federal investigators close in. If Trump offered a deal in return for Putin’s support on Iran, it would be seen as further evidence of the collusion he so vehemently denies. It would also finally kill any hope of a peaceful, democratic future for Syria.