The government has yet to schedule a vote on the future of Britain's independent nuclear deterrent, but the issue has already established itself as a key battleground between Labour leader Jeremy Corbyn and his enemies among the party's MPs.
With a decision due this year on renewing the four Vanguard submarines that carry the Trident nuclear missiles, Corbyn has initiated a review of Labour policy, which currently supports renewal.
Corbyn has campaigned for unilateral nuclear disarmament all his life, a position shared by many party members, but most Labour MPs support renewing Trident.
At a meeting of the Parliamentary Labour Party on Monday night, shadow defence secretary and Corbyn ally Emily Thornberry was heckled and jeered as she took questions about the policy review.
Kevan Jones, sacked by Corbyn last month as a defence spokesman, complained that Thornberry was "waffly and incoherent".
Madeleine Moon, a Labour member of the Commons defence committee, tweeted: "Oh dear oh dear omg oh dear oh dear need to go rest in a darkened room".
Thornberry provoked these splenetic outbursts by suggesting that, since the first of any new submarines will not become operational until 2028, Labour should consider the possibility that Trident could become obsolete over the next few decades.
Uniquely among the nuclear powers, Britain’s deterrent depends exclusively on submarines, one of which is constantly at sea, its whereabouts never disclosed.
Thornberry told MPs that Trident could be overtaken by technological advances. She cited underwater drones, which are being developed to track submarines.
“The idea of the Trident replacement is that it can hide in the sea,” she told the BBC. “If technology is moving faster than that, then it may well be that Trident may not be able to hide.
“If that’s right, and if we are to bet everything on “mutually assured destruction”, we have to be assured that it will work – and if it cannot hide any more, then that is a problem.”
Other options
Thornberry’s broader point was that the choice over Trident need not be a binary one – to renew it completely or scrap it – and that the policy review was an opportunity to consider other options.
Another Corbyn ally, Norwich MP Clive Lewis, later identified a few alternatives, including a move to land- based missiles, replacing Trident missiles with cruise missiles, and sharing a nuclear deterrent with France.
Labour’s decision is complicated by the fact that some trade unions representing those who work in the nuclear defence industry are determined that Labour’s support for Trident should not change.
Shadow home secretary Andy Burnham warned it may not be possible to reconcile opposing views on the issue.
“We always knew this was going to be a difficult debate for the party,” he said. “There are two positions here that are difficult to reconcile, maybe impossible to reconcile, and the party has got to find some way of accommodating those positions and move forward and not let this issue take over everything.”
Tory support
When
Tony Blair
decided to begin work on replacing Trident in 2007, he relied on Tory votes because 88 Labour MPs rebelled, among them former cabinet ministers.
At the time, some senior military figures and defence experts said Trident was too expensive and no longer useful as a deterrent in a post-cold war world.
The Labour rebellion that year would have been bigger had the Blair government not agreed to leave to a future government the decision on whether to renew the four Vanguard submarines, the most expensive part of the nuclear programme.
When David Cameron took office in 2010, he postponed the so-called Main Gate decision on Trident to 2016 to avoid a conflict with his Liberal Democrat coalition partners.
Cameron’s Conservative majority should ensure he wins a renewal vote in the Commons, but the issue will continue to be politically charged, especially in Scotland.
The 2007 decision to replace Trident came before the rise of the Scottish National Party (SNP), which has promised to evict the nuclear submarines from their bases in the Clyde if Scotland becomes independent.
If Cameron wins support in parliament for renewing Trident, he will do so with the support of just one of Scotland's 59 MPs – his own Scottish secretary.
The SNP MPs will all vote against renewal, as will the single Scottish Labour and Liberal Democrat MPs.
Pro-Trident Labour MPs believe that a unilateralist position will make it impossible for the party to win back votes in England.
Voting to renew Trident could make Labour’s challenge in Scotland just as difficult.