There’s a great Smiths track called ‘I Started Something I Couldn’t Finish’. Actually, I think all Smiths and Morrissey tracks are great, but that’s a column for another website.

That phrase sums up the approach taken by Jeremy Corbyn to the Syria vote. Starting an unnecessary fight and then having to climb down.

I don’t know if it is Jeremy himself strategising, or persons unidentified in his team, but they are progressively squandering the immense political capital that his big victory in the leadership election provided by issue-by-issue seeking to divide the Labour Party rather than build alliances.

The problem is that the electoral mandate you have as leader gets you so far, but it doesn’t give you presidential powers to run the party by diktat.

Where people agree with you, or at least don’t feel an issue is one they should die in a ditch for, you can deliver big changes in direction fairly easily – for instance sharpening Labour’s stance against austerity.

But when it comes to issues that people in your own party have deeply held but diametrically opposite moral views on, like the need for a strategic nuclear deterrent, or the case for air strikes against ISIL, you have to be a bit cannier.

You have to do the maths – can I possibly carry this vote?

You have to look at the rulebook – where does my authority come from, where does theirs come from, how is a disagreement resolved?

You have to look at the balance of political deterrence – if I push the people who disagree with me too far, can they do something dramatic (like resign from their job) that really damages me?

You have to look at the leverage you have over people. Can I offer them something they want to get their support? Basic transactional politics.

You have to take people with you. Persuade them. Compromise with them.

None of this seems to have happened in the run-up to yesterday’s Shadow Cabinet row over Syria.

Jeremy himself appointed the Shadow Cabinet. These are not a bunch of disgruntled rebel Blairite ultras, they are the people who volunteered to serve because they are team players, who wanted to try to make Jeremy’s leadership work. People like Tom Watson, Hilary Benn, Maria Eagle, Angela Eagle who sit squarely in the middle of Labour’s spectrum, or even slightly to the left of it. Who had all supported Ed Miliband in opposing the previous round of proposed air strikes but are thoughtful politicians who had looked at the evidence after the Paris attacks and gradually come to the reluctant conclusion this was necessary.

Jeremy presumably put Hilary and Maria in the key FCO and MoD roles because he trusted their judgement.

A simple head count last week would have told him there was a majority in the Shadow Cabinet he himself appointed for air strikes.

A quick flick through the PLP standing orders and a chat with Chief Whip Rosie Winterton would have confirmed that the rules say she derives the authority to whip Labour MPs not from the leader’s personal view but from a collective decision of the Shadow Cabinet – how can you have a doctrine of collective responsibility if you don’t take decisions jointly?

What leverage does a leader have over Shadow Cabinet members who are doing him a favour by serving under him in circumstances which in the last two weeks have been toe-curlingly embarrassing?

What leverage does he have over MPs when the threat of de-selection by local activists (a – insert obligatory Mao quote – “paper tiger” in any case as you would have to be stunningly inept at politics and organising as an incumbent to not secure a majority of branches and affiliates in a trigger ballot) is already massively trumped by the threat of electoral annihilation by centrist voters, given today’s poll showing we have lost 32% of the support we had in May?

Once the UN Security Council had passed its “all necessary measures” resolution about ISIL, an honest look at the Labour Party policy agreed only two months ago by Annual Conference would have revealed the conditions in it for supporting action (and remember it was written before the Paris attacks) had been met. Labour Party Marxists (a very, very small affiliate of John McDonnell’s Labour Representation Committee) has helpfully provided this account of the cynicism with which the Syria Emergency Motion was drafted by Campaign for Labour Party Democracy, based on a false assumption Russia and China would veto any resolution:

“Saturday saw the Campaign for Labour Party Democracy’s ‘Conference lift-off’ fringe. At this meeting Jon Lansman urged people to support the CLPD emergency motion on Syria, which sought to undermine the contemporary motion from Labour First by requiring any intervention in Syria to have a mandate from the United Nations. Stan Keable of Labour Party Marxists pointed out that the motion did not really oppose intervention, but simply placed conditions on it. Lansman retorted that the motion de facto ruled out intervention and had been drafted to ensure the widest possible support.”

Looking at all this evidence, the logical response would have been to avoid a fight the leadership could not win, and instead immediately go for a free vote to save face, avoid advertising the lack of a majority in the Shadow Cabinet for Jeremy’s position, and avoid further damage to party unity.

Instead there was a bizarre and damaging attempt to pick a fight using all kinds of heavy-handed unconstitutional methods, the basic subtext of which was implicit bullying: leave your conscience and your views on national security at the door or the boys will come round and deselect you. Public interventions from Len McCluskey; sham consultations of party members with voodoo polling methodology; email writing campaigns by Momentum.

An immense amount of good will was burnt over the weekend. Shadow Cabinet members were made to feel their role is a sham and their views are held in contempt. People who had taken a political risk to do the right thing by serving under Jeremy were harassed by online trolls. MPs who were actually pretty sceptical about air strikes and open to persuasion about Jeremy’s case have reacted like anyone with any guts would when confronted by a bully.

It takes quite a lot to start a political fight with, and allegedly provoke a threat of resignation from, a man as conciliatory, mild-mannered and collegiate as Hilary Benn.

It takes quite a lot to get Margaret Beckett – who ran against Blair as the left candidate in 1994, denounced Kinnock for not backing Benn in 1981, nominated Corbyn – to allegedly tell the PLP: “We cannot unite the party if the leader’s office is determined to divide us”.

A free vote was always the best Jeremy could have hoped for. He is extremely lucky that the Shadow Cabinet majority did not use their powers to whip him and the rest of the PLP to support air strikes. Instead of a free vote being offered magnanimously in a move that has would have been seen as statesmanlike, inclusive and unifying, it had to be wrung out of him after a weekend of acrimony and bad headlines that has done yet further damage to our party’s reputation and our electoral chances in the Oldham West by-election.

Oddly, Jeremy’s close ally John McDonnell seemed to get this, and was consistently arguing for a free vote. Did they fall out or was this some kind of “good cop, bad cop” act?

This is the second time this attempt to pick unnecessary and unwinnable fights on issues of national security has nearly derailed Jeremy’s leadership, the first being the attempt to force a vote on Trident at Annual Conference.

Each one weakens party unity and weakens Jeremy’s authority.

A third time may not be survivable.