Something remarkable happened in British politics on Wednesday; something that has not happened for a generation. The leader of the opposition, Labour’s Jeremy Corbyn – who would, as things stand, become prime minister if Labour were to win the 2020 general election – said that there were no circumstances under which he would authorise the use of nuclear weapons.

In doing so, he accelerated the arrival of a confrontation that some had predicted would occur next year, when Parliament votes on whether or not to proceed with the renewal of Britain’s Trident-based nuclear-weapons system. Jeremy Corbyn is resolutely opposed to Britain’s possession of nuclear weapons, even though Labour’s last five election manifestos have said that Britain should retain its arsenal until such time as global disarmament becomes realistic. That policy was effectively reaffirmed this week, when Labour decided against debating Trident (as the programme as a whole is colloquially known) at its annual conference.

Corbyn’s position upon becoming party leader was that he would not insist on support from his shadow cabinet on this issue, only that they should be willing to engage in debate. But the party appears collectively to have resolved – spurred by the trade unions, which view Trident as a valuable source of industrial jobs – not to revise its stance, at least for the moment.

The confrontation is this: Corbyn’s opposition to the renewal of Trident is one thing, and is in theory an issue on which a dispute between him and the rest of his party could be workable. But Corbyn’s opposition to the use of nuclear weapons, and, crucially, his willingness to make this explicit in public, is an unbridgeable gap.

To understand why, consider this scenario: in 2016, Corbyn permits a free vote amongst the parliamentary Labour party on Trident renewal. The majority of the party votes in favour of Britain retaining the nuclear deterrent; Corbyn and some of his allies join the Scottish National Party and some Liberal Democrats in voting against. The Conservatives vote as a bloc in favour and, commanding a parliamentary majority in any case, win decisively. The work to build four new nuclear-armed submarines begins. Now let us imagine that Corbyn is still Labour Party leader in the run-up to the 2020 general election. If he has not managed by then to change the party’s collective stance, he will be stuck: Britain will be building a nuclear-weapons delivery system that the prospective prime minister has declared he is not willing to use.

Jeremy Corbyn is not the first Labour leader to be on the record as opposing nuclear use – both Michael Foot and Neil Kinnock at various points were similarly resolved. But those leaders had the backing of the party; and when Kinnock’s position later changed, the shift was over a long enough period to make his commitment to nuclear deterrence somewhat believable. For Corbyn, things are different: if he were to campaign for the 2020 general election saying that he had reconsidered, who would believe him? Thousands of pages have been written about the difficulties of communicating credibility in deterrence. But what happens when a nuclear-weapon state’s leader has said he will not threaten nuclear retaliation at all?

All of which explains why Jeremy Corbyn’s words on Wednesday – even though perhaps unsurprising from a lifelong advocate for disarmament – were so remarkable. Before then, it was just about conceivable that Corbyn could have been a reluctant nuclear deterrer: personally opposed, but pledging to accept the judgement of his party and of parliament. Now that escape – which would have been a high-wire act anyway – is out of reach. In response, Maria Eagle, Corbyn’s own shadow defence secretary, mustered up a piece of British understatement: ‘I don't think that a potential prime minister answering a question like that, in the way in which he did, is helpful’.

Matthew Harries is Managing Editor of Survival, and a Research Fellow at the IISS. He can be followed on Twitter at @harries_matthew.

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