When the people of the future look back at our time, there will be much wringing of hands at the west’s failure to stop the slaughter in Syria. Liberal writers will bewail our “guilt” and “shame” (bewailing is what we liberals are best at, after all). Readers will pat themselves on the back and say that they would never have behaved as we behaved; just as we look back on the Second World War and imagine we would never have collaborated if the Nazis had invaded.

Look at what the generation of the 2010s ignored as they admired their iPhones and took their selfies, they will say in shocked voices.

By 2015, the Syrian civil war had lasted longer than the First World War. Hundreds of thousands had died and 11 million had been driven from their homes, four million of them as refugees to foreign lands.

On the one side was Bashar al-Assad, chief capo in a hereditary tyranny. He joined Saddam Hussein in becoming one of only two leaders to have used chemical weapons against civilians since the end of the Second World War. In 2013, Barack Obama, the leader of the free world, no less, boomed: “What kind of world will we live in if the United States of America sees a dictator brazenly violate international law with poison gas and we choose to look the other way?” He then looked the other way. As did the British Labour party, which joined with the Tory right in defeating David Cameron’s attempt to punish Assad.

On the other side is Islamic State. If you want a comparison to shame you, consider that at least 700 religious fanatics have left Britain to rape, murder and enslave in Syria at a time when the British government was pulling every trick it could think of to stop Syrians fleeing rape, murder and enslavement finding asylum here.

Here is a taste of the condemnations we can expect from the future. Western leaders interpreted the “silence” about the massacres in Syria “as an indicator of public indifference”. They reasoned that they “would incur no costs” if they did nothing, but “face steep risks if they intervened”. For all their cries of “never again”, they accepted genocide and pretended it had nothing to do with them.

Except those quotes are from the past not the future. They come from Samantha Power’s A Problem From Hell: America and the Age of Genocide, a bleak account, published in 2003, of how from the Turkish massacre of the Armenians through to Saddam’s genocide of the Kurds, the unwritten rule of the US State Department was that America should look the other way.

I was hugely impressed by her breadth of scholarship, how she showed that it was always unpopular to state clearly that Armenians, Jews, Cambodians, Kurds, Bosniaks and Tutsis were victims of the greatest of crimes; how there were always authoritative voices warning us against “overreaction” and insisting that the situation was more complicated than it seemed.

Those who blew the whistle lost their jobs but in their determination to speak out, they proved the truth of George Bernard Shaw’s maxim: “The reasonable man adapts himself to the conditions that surround him... The unreasonable man adapts surrounding conditions to himself... All progress depends on the unreasonable man.”

Obama was impressed too. He made Power America’s UN ambassador, where together they became reasonable – depressingly, shamefully, criminally reasonable – and stood by as the Syrian massacres escalated.

For understandable reasons, a half-truth became established during the presidency of George W Bush: America was the main source of conflict on the planet. If it stepped back and refused to intervene, the “root cause” of violence would vanish. Obama and Powers have tested that theory to destruction.

They have shown that, when the west does not intervene, other powers do. Russia and Iran have ruthlessly pursued their national interest in keeping Assad in power: Iran because it wants a client Shia state; Russia because it wants to keep its Mediterranean base and show the world that no one messes with Putin.

No one in the west, or, rather, no one but the reckless, wanted an invasion of Syria. They wanted no-fly zones and safe havens. A few realised that the Kurds had as much right to a state as the Palestinians and wanted western support for a Kurdistan, not least because the Kurds were doing most of the fighting against Islamic State.

They’ve got nothing. The Kurds are now being attacked by our Nato ally Turkey. Safe havens remain a fantasy. And while western air forces are bombing Isis in a desultory war that seems to be doing no good at all, they allow Assad to drop barrel bombs on Syrian civilians. Although Cameron has behaved more honourably than Labour, and has clearly agonised over the Syrian crisis, his agonies have not extended to providing the money our dilapidated armed forces would need to intervene.

Before power made her “reasonable”, Samantha Power knew why: western electorates don’t care. The mood in Britain in particular is now isolationist: anti-immigrant, anti-intervention, anti any measure that does not put “our own people first”. I see no sign that the flood of refugees fleeing into Europe is changing minds. Liberals rightly criticise Cameron for not allowing enough into Britain, but hardly any have shown that they have the smallest inclination to tackle the “root cause” of their flight.

Now western governments hint that they are about to commit the final treason. They will either drop their demands that the butcher Assad must go or, more probably, quietly accept that he is a man they must do business with.

There’s an old argument between supporters of an ethical and of a “realist” foreign policy, but it does not arise on this occasion. The Sunni people of Syria will not turn on Isis so they can suffer again at the hands of a man responsible for gassing their families. Isis will be able to say – with justice – that the west wants to turn you over to Shia, Hezbollah and Iranian militias. It will be able to say, again with truth, that the west is now the de facto ally of an Iran that wants to encircle and oppress you.

Sometimes, the ethical is also realistic: dealing with Assad is never going to work.