After the military corrupted the English language with “collateral damage”, I’d like to introduce the equally dainty and equally misleading “collateral benefit”. I hope you like the smooth way the euphemism oozes from the lips; the imperceptible subtlety with which it shuffles off responsibility.

The phrase implies, without being so crude as to say so out loud, that the west does not intend mass murderers to benefit from its wars any more than it intends civilians to die in its airstrikes. If when the accountants of violence make their reckoning, the dictators are as triumphant as the civilians are dead, that is no concern of ours.

Bashar Assad is now enjoying the collateral benefits of western foreign policy. It is not that he, and by extension Iran and Hezbollah, is our formal ally. We still have our standards, after all. If their power is strengthened, and the bombing and slaughtering of civilians continues, we regret it, naturally. These are unintended side-effects no one can expect us to control.

Human suffering is not a competition. You can’t measure mounds of corpses and reserve your criticism for the highest. Yet when Barack Obama addressed the UN, he did not even glance at the mountain of bodies in Syria. He described the war crimes of Islamic State, but did not once say that clerical fascism had been nurtured by the bloodier war Assad had launched against the Syrian version of the Arab spring.

“We will support Iraqis and Syrians fighting to reclaim their communities,” Obama cried. But only if they were fighting to reclaim them from Islamic State.

Between 2011, when peaceful demonstrators demanded the removal of a Ba’athist dictatorship that has tyrannised Syria since 1963, and April this year, the UN said that 191,000 people had been killed – the figure is “probably an underestimate”, it added. About nine million Syrians have fled their homes. To comprehend the catastrophe the Assad regime has brought, you must imagine an apocalyptic Britain where the entire population of London – and then some – run for their lives. Assad has launched chemical weapons attacks on the suburbs of his own capital. The gallant Syrian air force has dropped incendiary bombs on school playgrounds. Uncounted thousands, including relief workers, lawyers and doctors, have disappeared into his prisons where their jailers have beaten, mutilated and raped them.

Obama might have thrown every condemnation he threw at Islamic State at the Assad regime. Both have “terrorised all whom they come across” in Syria. Both have subjected “mothers, sisters and daughters to rape as a weapon of war”. Both “have gunned down innocent children”. But while Obama said Islamic State had shocked “the conscience of the world”, he could not manage one word about Assad.

I accept that the conscience of the world is as flexible as an iPhone. And I have mentioned before how Mr Obama’s bends with the wind. But his behaviour, and that of the wider west, remains extraordinary. We are going to war against a barbaric enemy, but no one is talking about the barbarism that helped create it. That airstrikes against Assad’s enemies must strengthen his chances of survival is not a fit subject for discussion.

I am tempted to write that Obama’s willingness to aid criminals is Nixonian. Authorities on the Middle East are already looking at the diplomatic exchanges with Tehran and speculating that he is edging towards his own Nixon in China moment. Nadim Shehadi of Chatham House says that Assad must be waiting for the news that Obama is prepared to allow him to dominate Syria and his Iranian puppetmasters to dominate Iraq and Lebanon as well. Perhaps, however, the comparison with the worst of his predecessors is too kind to Obama. Nixon and Kissinger would do anything and support anyone who was against the Soviet Union. For all their crimes, they had a brutal singlemindedness. I struggle to find coherence of any kind in Obama’s foreign policy.

Assad does not care. He knows he is winning, whatever the president’s motives. Ali Haidar, his “minister for national reconciliation”, purred like the Queen on hearing the result of the Scottish referendum when he described the US attacks on Islamic State targets. “What has happened so far is proceeding in the right direction in terms of informing the Syrian government and by not targeting Syrian military installations.”

I am not disputing the need to confront Islamic State. Militant Islam will drive the Christians and Zoroastrians out of Iraq like the Jews before them. The Kurds, who could be our truest friends in the region, may suffer yet more massacres. Britain has a particular moral responsibility to confront radical Islam when our “vibrant” and “diverse” society has furnished Islamic State with so many willing executioners. (Or would have a moral responsibility if our armed forces had not lost their battle with George Osborne and been left in no condition to confront anyone.)

But just as the west won’t recognise the right of the Kurds to self-determination, so it won’t accept that you cannot fight Islamic State in Syria without offering hope to those who oppose Assad. Instead it carries on propagating the authentically orientalist lie that Syrians are either Ba’athists or Islamists, and there is no alternative to tyranny.

I do not believe the line will hold. How long will the Sunni Arab states stay in Obama’s coalition when they see their Shia enemies benefiting? How will Syrians react to the double standard?

It is as if every liberal chant of the last decade is returning to haunt Obama. You will remember hearing, or perhaps said yourself, that we should concentrate on the “root causes” of terrorism. The root cause of Islamic State is Shia sectarianism in Iraq and sectarian mass murder in Syria. Mohammed Antabli, a leader of Syrian exiles in Britain, told me how he took British and European politicians to Turkey’s border with Syria at the start of the war and warned them that Islamism would flourish if the west did nothing for the moderate opposition. And so it has.

You will remember hearing, or perhaps said yourself, that we must not alienate “the Arab street”. Kassem Eid, an opposition activist now in exile in the US, said what streets were left in Syria were alienated beyond measure. The democratic world has done nothing. No no-fly zone. No attempt to slow Assad down, even for a day, even when he crossed Obama’s “red line” on chemical weapons. Every Syrian activist I spoke to repeated his assertion that western hypocrisies were driving support for Islamism.

In his 1 September, 1939, on the eve of another war, WH Auden wrote: “I and the public know/ that all schoolchildren learn,/ Those to whom evil is done/ Do evil in return.”

A great evil has been done to Syria. I cannot see how any western project against Islamic State can prosper until the “conscience of the world” provides redress by saying it will not tolerate the continuation of the Assad regime. At present, however, the world won’t even acknowledge evil’s existence. We must expect evil in return.