So it’s war, then. Tom Watson has announced the end of talks about an amicable settlement between Jeremy Corbyn and Labour’s rebels. Angela Eagle, an ever-present on Labour’s frontbench since 2007, has announced that she will challenge Corbyn for the leadership.

What happens now?

The fix may still be on. Corbyn may not make it to the ballot

The Labour party is a bit like a post-Soviet state: it has a democratic constitution, but not a democratic culture. The somewhat vague wording of the party’s rules means that it is up in the air whether or not Corbyn, or merely his challengers, will need to seek the support of 20 per cent of the parliamentary Labour party (that includes not just his colleagues at Westminster but the 20 members of the European Parliament as well).

There are competing bits of legal advice, and the precedent in 1988, when Tony Benn challenged Neil Kinnock (with the support of Corbyn among others) is that the leader had to seek nominations. My understanding is that GRM Law, the party’s official lawyers, advise that Corbyn would need to seek nominations, while Doughty Chambers, who were commissioned by the leaders’ office and whose legal advice has been well-leaked, say that he would not.

But in the end what will matter is what the party’s ruling national executive committee decides to do. Certainly, some of the rebels have been alive to the possibility that Corbyn may need nominations to stand, with informal talks taking place in both the parliamentary party and the European parliamentary party.

I’m told that the somewhat abrupt ending of negotiations by Watson took some of the trade unions by surprise, and the BBC’s Iain Watson, one of Westminster’s most experienced Labour-watchers, has heard the same. It may be that Watson is confident that he can keep Corbyn out of contention entirely.

If he is on the ballot, he starts as the heavy favourite

I said on the BBC last week that Corbyn either had a zero per cent chance of competing or a 100 per cent chance of winning. Having spent the back end of last week talking to Labour activists, it is clear to me that he remains well-placed to win.

Although there is significant disquiet with Corbyn’s performance, particularly around competence, among Labour members, the grassroots are reluctant to change leaders as they fear that it will mean a return to the days of anti-immigration mugs and lukewarm opposition to the Conservative cuts.

That both the candidates for the Conservative leadership – and indeed, George Osborne himself – have abandoned Osborne’s fiscal targets in the wake of Brexit means that the economic dividing line will not be a factor as both Corbyn and his rival will run on a ticket of fiscal stimulus – housebuilding, transport projects, and green energy will likely be the big themes.

But Angela Eagle voted for the Iraq war. Though she has since U-turned, if she is the candidate, Corbyn’s campaign will be a noun, a verb and the Chilcot report. Team Corbyn will weaponise that vote to talk about their candidate's voting record and history of left-wing campaigns. (They will skate over the consistent opposition to the European project.) Corbyn will triumph and it won’t be close.

That's why some Corbynsceptics are rallying around Owen Smith, who may yet also run. But Smith has suggested that there may be a “progressive case” against free movement, making him vulnerable on immigration. That also disqualifies him from running as an unapologeticaly pro-European candidate, probably the only dividing line that rebels can draw over which they emerge stronger than Corbyn.

It will be difficult for any “unity candidate” to defeat Corbyn

The phrase “unity candidate” is an unfortunate codeword for “candidate who no one in the parliamentary party regards as sufficiently formidable to be leader in a decade’s time”.

Wavering supporters of Corbyn want to be inspired – but any candidate impressive enough to do that is unlikely to be acceptable as a “unity” candidate. The candidates who Corbyn would have reason to fear – from talking to members, Keir Starmer is the most lethal, though there are others – cannot run without triggering a full-blown, multi-candidate contest, which the PLP believes would hand victory to Corbyn.

There is a narrow path to victory even for a unity candidate

Although I would be fairly surprised if Corbyn were to be defeated, there is a plausible if thin path to victory for his opponent, even if it is Eagle or Smith.

As I’ve written before, the rebels have come relatively late to effective social media organising, in stark contrast to the larger and more established presences of Red Labour and other groups on Facebook. Momentum, meanwhile, has a far bigger presence from a standing start than any of the anti-Corbyn groups and particularly Saving Labour, the new anti-Corbyn group.

But despite their relative lack of preparation, the possibility for Saving Labour to gain traction despite itself remains a real one (they have the advantage of having the support of many high-profile Labour supporting celebrities, including the actors Jason Isaacs and Frances Barber and the author JK Rowling).

The relative lack of dynamism of Eagle and Smith might even help as they are something of a blank canvas for a “save the party” message. It could be that the existence of a candidate, even a bad or bland one, galvanises a recruitment drive.

That said, I wouldn’t bet on it. If he makes it to the ballot, Corbyn’s chances of winning might not quite be 100 per cent. But they aren’t far off.