The ceasefire in Syria may not have been formally pronounced dead, but hopes to resurrect it are fast dwindling. After an aid convoy was destroyed near Aleppo, fighting again intensified and the US and Russia exchanged accusations in the UN. But in reality US diplomacy had collapsed before these latest events.

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Last week, just hours after western coalition airstrikes mistakenly targeted Syrian government forces, killing more than 60 people, the US ambassador to the UN, Samantha Power, made an extraordinary statement that served to highlight the contradictions at the heart of the Obama administration.

Power lambasted Russia’s “uniquely cynical and hypocritical stunt” for having convened an emergency UN security council meeting over the bombing of Syrian troops. She lashed out at how Russia had, over the past five years, consistently propped up the Assad regime and protected it from any consequences of its murderous policies. At length, she described Bashar al-Assad’s strategy of “death by a thousand paper cuts”: starvation sieges; the “horrifying, predictable regularity” of strikes on civilian targets; the “routine” use of chemical weapons; and “torture chambers” holding “tens of thousands of people”. Why, she asked, had Russia never once called an urgent security council meeting over such horrors?

There have long been two takes on Syria. One is the geopolitical realism line, which Barack Obama has chosen to follow largely because it fits with his reluctance to get involved in another war. The line is that US or western security interests are not at stake in an intractable, far-flung civil war that can more easily be contained than solved. The other is the moral imperative line that Power has repeatedly advocated within the administration. It refers to the doctrine of “responsibility to protect”, according to which a state’s sovereignty can be violated when a regime slaughters its own citizens.

Power published a book in 2002, A Problem from Hell, describing how US governments had historically failed to prevent genocides and mass atrocities – the book reportedly drew Obama’s attention to her when he was a senator.

The differences between Power and Obama were apparent in her stinging UN statement. By contrast, Obama said little on Syria in his UN speech this week.

In his 2009 Nobel prize speech, Obama said that inaction in the face of mass slaughter “tears at our conscience and can lead to most costly intervention later”. As Syria turned into hell on earth, the president repeatedly made the case that any intervention would be either futile or dangerous.

In tune with the American public’s aversion to military involvement after the disastrous Bush years, his policies have ranged from attempts to negotiate Assad’s physical departure from Syria (a plan dubbed the Yemeni scenario in 2012) to creating a “hub” in Turkey where rebels would be armed and trained (but without anti-tank or anti-aircraft weaponry), and – once Russia’s military intervention was launched in 2015 – more intense diplomacy with Moscow.

Results have been scant. What seems to have been missing from the US calculus, and what Power’s comments have perhaps unwillingly underscored, is that so many signs point to a crude reality: that Assad and his backer, Vladimir Putin, aren’t just indifferent to crimes against humanity but believe they serve their purpose.

Facebook Twitter Pinterest US ambassador Samantha Powers with secretary of state John Kerry at the security council at the UN in New York on 21 September. Photograph: UPI / Barcroft Images

Assad warned back in 2011 that he would spread chaos throughout Syria and beyond if calls for him to step down weren’t retracted. Putin has constructed his whole domestic political narrative around the notion that, however much he may be criticised by the west, he has restored Russia’s strength. Nor is Putin much bothered by the crimes of the Assad regime – his own army carried out similar massacres in Chechnya.

It’s possible Power’s statement was aimed at western public opinion rather than at the autocrats who, time and again, have demonstrated their capacity to order or tolerate untold levels of violence against civilians.

As his presidency comes to a close, the fact is that Obama has little to show the world on Syria. With an estimated half a million deaths, the Middle East in flames and European allies destabilised by the impact of refugee flows, he will pass on a festering crisis to his successor.

Russia was always going to be a stumbling block, not least because Putin long ago identified Obama’s reluctance to do more – such as arming the rebels decisively, upholding his self-proclaimed “red line” or setting up a no-fly zone (before Russian intervention made that impossible). There is a long list of missed opportunities that might have forced Assad to the negotiating table.

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Earlier this year the Atlantic magazine quoted Obama saying of Putin: “The notion that somehow Russia is in a stronger position now than before he had to deploy military forces to Syria is to fundamentally misunderstand the nature of power in foreign affairs.” Yet today Putin does appear stronger, if only because the US finds itself scrambling to salvage the ceasefire deal without any leverage. Trying to blame and shame Russia over the UN convoy bombing is unlikely to achieve much on its own.

A key problem with the ceasefire deal was the plan to set up a US-Russia “joint implementation centre” to coordinate strikes against Islamic State. This was meant as an incentive, as Putin had long sought to be accepted as a coalition partner alongside the United States. But if implemented, such a coalition could make the US complicit in Russian airstrikes, which have been designed to strengthen Assad. The US would endorse a Russian intervention premised on the notion that there are only two actors in Syria: Assad and the jihadis.

Yet as Power pointed out, “the Syrian government, which bills itself as a fighter against terrorists, allows Isis to grow and grow and grow … Assad’s antics – his tactics, his strategy – have been a gift to terrorists in Syria and well beyond”.

By siding with Russia, the US would risk offering another gift to terrorists. Of course, American policymakers were well aware of this, which is why they set preconditions: a partial freezing of the civil war, and safe passage for humanitarian aid deliveries to Aleppo, the armed opposition’s last stronghold. But appealing to Putin’s sense of decency to get Assad to curtail his bombing was always going to be a gamble. Why would Putin seek to please the US once he had become an equal player alongside the sole global power?

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Obama has been in a hurry to reach a breakthrough on Syria these past few months, because he’s leaving office. Putin certainly isn’t. Nor is he constrained by domestic public opinion to do the right thing, or even try to.

The Obama administration is hanging on to the last shreds of its ceasefire plan – for lack of an alternative, though its critics would do well to avoid schadenfreude. With Isis-connected terrorism spreading in many parts of the world and illiberalism growing in Europe on the back of the refugee crisis, Syria has become a security threat well beyond the Middle East. Against that backdrop, geopolitical realism and moral imperatives look less like opposites.

Assad is capitalising on the balance of forces Russia has helped create. This in turn – as Power has said – will fuel Isis, whose twisted claim is to be the champion of Sunni Muslims. This means more problems lie ahead for the west, and more suffering for the Syrians.

Meanwhile, Putin is celebrated by populists around the world for having outmanoeuvred the US by pulling himself up to the ranks of a leader whose cooperation is almost begged for. Russia may be in recession, and its economy the size of Australia’s, but in Syria it has been given a free hand. Whatever one may think of American power and its limits, and however one may choose to gauge Syria’s importance to narrowly defined US national security interests, history will remember that this all happened under Obama’s watch.

A former adviser to Bill Clinton once reflected on how the US had failed to prevent massacres in Bosnia: he said that by 1995, “the issue had become a cancer on our foreign policy and on his administration’s leadership”. For this reason, Clinton in the end ordered targeted strikes on Serbian forces, which forced Slobodan Milošević to the negotiating table. Power, who covered that crisis as a journalist, eloquently told the story in her book.

No doubt Syria is a complex, multifaceted war, on a different scale to Bosnia. But the lesson that should still be drawn is that war diplomacy can only succeed if leverage is decisively built up, not just hoped for. If talking to Russia is the only gateway to a peace process in Syria, more than simple persuasion must be brought to bear on it.