On the face of it – and to the west’s excruciating embarrassment – Vladimir Putin is walking away with all the prizes in Syria. Our aim was to bring stability to Syria, but he is doing it. Our target was to remove President Assad. But Putin has ensured he stays. Our hope was to destroy Islamic State (Isis). But he is the only one with the force on the ground to make it happen.

Our presumption was that the days of Russian power in the Middle East were over. But today Russia has more influence from the north-east corner of the Mediterranean through to Iran than ever before. None of this is due to Putin’s genius. It is due to our follies.

It was folly for us to seek to remove Assad back in 2012 when it was not in our power to make it happen and wholly in Putin’s power to make sure it didn’t. It was folly to ignore the warnings at the time that this was not about Syria but about the beginnings of a religious conflict between Sunni and Shia, in which Russia and the west could be drawn in on opposing sides – which is precisely the danger that now so frighteningly confronts us.

It was folly for us to choose as our key Arab allies Saudi Arabia and Qatar, who were actually funding the jihadism we were trying to stop. It was folly to choose high explosive dropped from the air as our main – indeed, our only – instrument to achieve our aims, when Russia, with force on the ground, could outbid us, whenever they chose to – as they just have.

The road to hellish interventions is paved with solemn promises about 'limited engagements'

We have presented Putin with his openings on a plate and he has taken them with enthusiasm and panache. I suspect there is little we can do now to either restrain him or influence what happens on the ground, and I am not at all sure of the wisdom of even trying – at least in the short term.

But Russian triumphalism may be premature. Putin has his problems too. Recent Russian opinion polls show serious concern about being dragged into an Afghanistan-like quagmire. And justifiably so. As we too know to our cost, it is easy to start these things – much more difficult to end them.

What happens if, despite Russia acting as Assad’s air force, he still loses, perhaps not in whole but at least in part. Isis in the east and centre of the country is not falling back – it is still advancing. Having started with expansive promises to defeat terrorism, he could soon come face to face with the fact that he may not be able to. No modern leader takes his “face” more seriously. The road to hellish interventions is paved with solemn promises about “limited engagements” – a phrase Putin has used more than once.

Then there is the cost of all this, against the backdrop of a foundering economy and western sanctions – to say nothing of the real danger Putin now runs with increasing instability in the Caucasus in the Islamic republics of Dagestan and Chechnya.

No western leader will admit it, of course, but our military strategy in Syria has failed. We need to switch tack – back to what we should have been doing three years ago – a policy in which diplomacy, not high explosive, is the centrepiece.

What Syria will need in the end is a treaty-based Dayton-style regional agreement, supported by its neighbours and the great powers, which will underpin the territorial integrity and stability of the country. We should be building on the new rapprochement to draw Iran into the process. In the long run I suspect that Tehran understands it has more interest in building up its western relationship than in continuing to rely on an increasingly bankrupt and bellicose Russia.

If we left Putin to his bombing and outflanked him with diplomacy, we would probably achieve more than we can with yet more high explosive. And when in due course he has to find himself a way out, we can provide him with a ladder to climb down.

None of this is to suggest (unfortunately) that we can take our aircraft out of the sky – that would be a humiliation too far. We have chosen our bed and for the moment we must endure its thorns, not least because to do otherwise would embolden Putin further.

In fact, this is a moment when western solidarity is the best means to restrain his adventurism and capability for miscalculation. There is no military purpose to be served by Britain adding our widow’s mite of explosive to the mountain already criss-crossing the increasingly crowded Syrian skies. But there might be a political purpose, if it conveys solidarity. A Nato deployment of F-22 Raptors at Incirlik airbase in Turkey would have the same effect.

What all this amounts to is a twin-track strategy. Continuing with military action, while recognising that it will have little effect beyond illustrating to Putin that there are limits to his room for manoeuvre; but shifting our main effort to regional diplomacy, where we can now best outflank him. Then we can begin the task that we all know must be done one day – laying down the basis for the political solution without which Syria and its tortured people can never have peace.

Paddy Ashdown will be in conversation with Observer associate editor Andrew Rawnsley at a Guardian Members’ event on 10 November. Find out how to book tickets