Garvan Walshe is a former National and International Security Policy Adviser to the Conservative Party.

To come to an arrangement with Assad, who bombs, tortures and shells his countrymen by the hundred thousand. To accept Russian aircraft and air defence assets giving Hezbollah air cover, in Lebanon and near the Israeli border. To bully the rebels, as though they were pre-war Czechs, to come to disadvantageous terms with their tormentor. And to condemn people fleeing his terror to indefinite dependency in refugee camps. Without doubt an outrage, as Shiraz Maher and Nick Cohen have written so forcefully. But is it an outrage that can be justified?

Vladimir Putin, hands full with problems of his own making in Ukraine, steward of an economy battered by sanctions, and beset by falling commodity prices is now getting stuck into the Middle East. A detachment of troops to Syria here; an intelligence-sharing agreement with Baghdad there; and now, in front of the UN General Assembly, a proposal for talks to bring Syria’s civil war to an end and to fight ISIS. Has he been reading Ferdinand Foch? “My centre is giving way, my right is in retreat. Situation excellent. I shall attack!”

The West, too, finds itself in trouble. The refugee crisis is dividing Europeans: it splits towns and families as well as governments. An American scheme to train competent, reasonable, rebels has resulted in “four or five” men joining their ranks. And ISIS, though weaker than a year ago, is weathering the half-hearted Western air campaign aimed at containing it.

Assistance, it would appear, springs from the most surprising of sources. Yes, Putin has invaded Ukraine, seized Crimea, regularly buzzes Western airspace and has built up links with destabilising extremists of right and left across Europe, but here he is disposed to be helpful. Russia has even released Eston Kohver, the Estonian agent she kidnapped last year. To some, co-operation against ISIS, and of necessity with the Syrian Regime, begins to look attractive, if extremely grubby. To the more cynical Western descendant of Metternich, it could usefully entangle Moscow in a second bloody quagmire. There is business to be done at the UN this year.

Were this a low-grade movie, the ominous music would have begun to play, and our hero would find himself thinking: yes, I’ll walk down the darker of the two paths in the wood, where the carnivorous plants hiss and snap, and the bats circle overhead. Conservative Home studios are some time in the future however, so I’ll content myself with adapting Ronald Reagan: the ten scariest words in the English language should be “I’m from the Russian government and I’m here to help.”

Russia’s latest scheme, to join with the West in a campaign “against ISIS” that leaves Assad in place, has a major, obvious flaw: the Syrian regime is mostly fighting other rebels, and conducting indiscriminate bombing of rebel-held, not ISIS-held areas. His aim is not really to defeat ISIS, but to appropriate a bargaining chip through truculence, and strike at the West’s reputation as an ally.

In middle eastern capitals, the view is beginning to set in that we have turned fickle, are prone to emote, and are as likely to run away as to fight for an ally we are supposed to be backing. From Sunni tribal sheikhs in Iraq to Benjamin Netanyahu, there is a fear abroad that we are all too happy to cut our partners out when becomes inexpedient in the short-term to maintain them. Russia, though much weaker and limited to supplying military equipment that has repeatedly proved inferior to the West’s in battle, at least sticks by its men. Moscow understands that where international law doesn’t hold sway, political relations are feudal in character, and that constancy and loyalty count for a lot.

Were it to join in Putin’s new scheme, the West would look confused, unprincipled, hopeless and inconstant – as though its values (human rights, law and representative government, loudly professed) were to be kept guarded in North America and Europe as vital national secrets whose escape should be prevented. To ignore the Syrian civil war, as we have for almost five years, is abject and shameful. But to join Moscow in propping up its biggest murderer would be as foolish as it is evil.

The Prime Minister was right to say that Assad belongs in a war crimes court. But if he is serious about this, he needs to rally the world’s democracies in rejecting Russia’s corrupt initiative. This does not mean refusing to take part in negotiations, but in strengthening the moderate rebels and weakening the Assad regime as the negotiations go on so that, as in Bosnia, the disposition of forces assists in securing a better outcome from within them.

In this, he has been presented with a surprising gift from the Opposition. Hilary Benn has somehow bounced Jeremy Corbyn into calling for an international campaign to establish safe areas in Syria. This would be a significant military operation, not far off Kosovo in scale. The Chapter VII UN resolution he wants won’t happen, but Russia would be forced to veto it, and Kosovo itself provides the precedent by which, with the support of France and Germany as well as states in the region, sufficient legitimacy could be obtained for a mission to protect Syrian civilians from Assad. Sometimes help comes from surprising quarters indeed.