On Friday, near Palmyra, 14 tanks and an anti-aircraft system were destroyed in an air strike on Isis. Palmyra recently fell to the jihadists after the Syrian regime and its allies diverted forces to Aleppo, leaving the ancient city under-defended.

This was a repeat of events last year when, on the advice of the Iranian general Qassem Suleimani, the regime deployed troops away from Palmyra to the strategically significant metropolis of Aleppo. The planes struck Palmyra on the same day Suleimani was photographed treading the city’s rubble. But the planes weren’t Russian or Syrian: they belonged to the US-led international coalition. While the US has its own reasons for battling Isis, in this case it was picking up the slack from the regime.

Palmyra has only symbolic significance for Assad. Aleppo was the prize, and, with the world watching impotent, the regime was able to starve and bludgeon its population into surrender. The regime was aided by Russian bombers and special forces, Iranian Revolutionary Guards, Hezbollah mercenaries, and a horde of sectarian militias from Iraq, Afghanistan and Pakistan – but, above all, it was aided by American indifference.

In an interview with Jeffrey Goldberg of the Atlantic magazine earlier this year, President Obama said he was “very proud” of the moment in 2013 when, against the “overwhelming weight of conventional wisdom”, he decided not to honour his own “red line”, allowing Assad to escape accountability for a chemical attack that had killed more than 1,400 civilians.

Obama may be alone in this judgment. A year earlier, seemingly on a whim, he had set a red line on the use of chemical weapons at a time when none were being used. The red line was, in effect, a green light to conventional killing. But the regime called Obama’s bluff – and, predictably, he backed down. No longer fearing punishment, the regime escalated its tactics.

Nearly four times as many people were killed in the two years after the chemical attack as had died in the two years before. Obama’s abandonment discredited Syria’s nationalist opposition and empowered the Islamists. It helped Isis emerge from the shadows to establish itself as a major force. Together, these developments triggered a mass exodus that would displace over half the country’s population. And as the overflow from this deluge started trickling into Europe, it sparked a xenophobic backlash that has empowered the far right across the west.

These, however, weren’t the only consequences of Obama’s retreat. The inaction also created a vacuum that was filled by Iran and Russia. Emboldened by his unopposed advances into Ukraine and Syria, Putin has been probing weaknesses in the west’s military and political resolve – from provocative flights by Bear bombers along the Cornwall coast to direct interference in the US elections.

A man praying near the Umayyad mosque in the ruins of Aleppo. Photograph: STR/EPA

The post-second-world-war international order is on the verge of collapse. In January, when Obama leaves office, he will be leaving the world a lot less stable than even his predecessor.

But in his valedictory press conference, last Friday, Obama defended his policy on Syria – albeit with logic whose fractures even his eloquence could not conceal. Inverting cause and consequence, he cited Russian and Iranian presence in Syria as his reason for not confronting Assad (neither was there in August 2013); he cited the disunion among rebels as the reason for not supporting them (they fragmented because they were denied meaningful support); and he cited the fear of deeper American involvement as his justification for restraint (even though a year later it would lead to a far bigger deployment across two states).

The administration’s response to the neoconservative depredations of the past decade was to revert to old dogmas: the dogmas of “realism”. Under the influence of doctrinaire realists, Obama concluded that the Arab world was not ready for democracy; it needed “strongmen”. The strongmen would protect the west against the twin threats of terror and migration. This logic led the US to back Nouri al-Maliki’s sectarian government after the controversial 2010 election in Iraq; it also led it to tolerate Assad. Syria was defined narrowly as a counterterrorism problem.

But there was also another reason for tolerating Assad. The administration had gambled its reputation on the Iran deal – a deal whose success would distinguish Obama from his belligerent and quixotic predecessor. Iranian leaders, however, understood that by investing his legacy in the deal, Obama had also made himself its hostage. He couldn’t make too many demands for fear of undoing his own legacy. Conscious of this, Iranian hardliners saw no cause for constraint. Flush with cash from the deal, they have embarked on a foreign policy far more intransigent than anything Iran has pursued in a century. Tehran has little need for nuclear weapons when it can conquer Aleppo without them. And, as a client of Iran, Assad has enjoyed impunity.

Aleppo fell on Obama’s watch. He did not raise a finger to save the city even though he had mobilised America’s vast military assets on short notice to defend Kobani and imposed a no-fly zone over Hasakah. By withholding leverage, Obama also allowed Russia to use the charade of diplomacy to aid Iran and the regime’s military conquest.

In January, as Obama surrenders the White House to America’s own strongman, the Iran deal will probably not survive long. Its benefits were already made doubtful by Obama’s invertebracy. It is Aleppo that the world will remember him by.

Dr Muhammad Idrees Ahmad (@im_pulse) is author of The Road to Iraq: The Making of a Neoconservative War (Edinburgh University Press, 2014)