During the 1987 general election campaign, Neil Kinnock was asked by David Frost how he would respond were the United Kingdom threatened by a nuclear-armed Soviet Union. The Labour leader, then an advocate of unilateral disarmament, replied: “This is a classical choice between exterminating everything you stand for and the flower of your youth, or using all the resources you have to make any occupation totally untenable.”

Margaret Thatcher ruthlessly exploited his remarks, deriding the party’s policy as one “for defeat, surrender, occupation and, finally, prolonged guerilla fighting”. Kinnock protested that his comments had been distorted – “There is no question of guerilla warfare or a Dad’s Army” – but the charge stuck. Labour was irrevocably branded as “soft on defence”.

When Jeremy Corbyn remarked two days after the Paris attacks that he was “not happy” with a “shoot-to-kill” policy against terrorists, it was the 1980s that MPs recalled. Shadow cabinet ministers suggested that it was the killing of three unarmed IRA members in Gibraltar in 1988, rather than the present threat of Islamic State, that shaped his thinking.

It was Corbyn’s MPs, not the Conservatives, who forced a clarification. A few hours after he made his remarks, Corbyn attended what several of those present described as the most extraordinary meeting of the Parliamentary Labour Party they had witnessed. "I've never seen the leader shouted over before," one told me. Corbyn was challenged over multiple issues but shoot-to-kill most of all. The following day, after failing to win the backing of the shadow foreign secretary, Hilary Benn (“I can’t speak for Jeremy”), Corbyn stated in his leader’s report to the National Executive Committee: “Of course I support the use of whatever proportionate and strictly necessary force is required to save life in response to attacks of the kind we saw in Paris.”

But as in the case of Kinnock, it is the original quote that Labour MPs worry will endure. The former cabinet minister Ben Bradshaw told me: “The first responsibility of any government is the protection of its citizens and the defence of the realm. Trust in Labour on these issues was hard-won in the Eighties and Nineties and any equivocation on such vital matters is unhelpful.”

Corbyn’s riposte is that far from preserving national security, the policies pursued by the Conservative government and its predecessors have undermined it. In a speech that was postponed after the Paris attacks, he was due to say that “a succession of disastrous wars” had “increased, not diminished the threats” to the UK. Even some MPs who share this view fear that he will not win a hearing for such arguments after his recent statements.

Outraged emails from voters and Labour Party members followed his comments on shoot-to-kill, they say. In their constituencies, MPs are asked how a leader who has declared that he would never use the nuclear button can defend the nation. In the run-up to the Oldham West and Royton by-election in December, Ukip, which has sought to turn the contest into a referendum on Corbyn’s patriotism, is profiting, or so some believe.

Should David Cameron table a motion for air strikes against Isis in Syria, Corbyn will soon face the greatest test of party unity to date. After previously appearing to relinquish hope of winning Commons approval, the Prime Minister has been emboldened by a combination of the Paris massacres, pressure from Barack Obama and the possibility of an agreement with Russia.

“There’s definitely been a change in the mood,” Johnny Mercer, the Conservative MP for Plymouth Moor View and a former soldier, told me. “Part of protecting ourselves is pursuing air strikes. I think there’s a majority there, there’s a clear majority.” Daniel Kawczynski, a Tory member of the foreign affairs select committee, told me: “A lot of MPs who are sitting on the fence may not want to appear soft on [Isis], bearing in mind it could be London that’s targeted next.”

Having previously refused to rule out a free vote for Labour MPs on air strikes, Corbyn has now come close to doing so. The announcement was the first evidence of the more forthright approach he promised at a recent shadow cabinet meeting. Yet, because of his record of rebellion, few backbenchers will join him in opposing air strikes out of loyalty alone.

Meanwhile, Benn is growing in influence. Unlike the Labour leader, who has rejected the question of whether he would ever support the use of force against jihadists as “hypothetical”, the shadow foreign secretary told the PLP meeting: “To those that say that taking action in Syria will make things worse: I say things are pretty bad for those in Syria and for our citizens, too.”

For David Cameron, the task is to persuade sceptics that he has what he lacked in 2013: a coherent strategy for Syria. Conservative sources are hopeful that as many as 30 Labour interventionists will counterbalance a maximum of 20 Tory rebels.

In his favour, Corbyn can cite a Survation poll conducted following the Paris attacks showing public support of just 15 per cent for independent UK air strikes on Isis in Syria. But though MPs acknowledge that many of his individual policies are popular, they fear that Labour’s collective offer is no longer “credible”.

For this reason, the question of deselection, be it of Jeremy Corbyn or of recalcitrant MPs, will persist. True unity will not be achieved until the PLP reflects the leader, or the leader reflects the PLP. “It’s pretty nasty. This is not the fraternal Labour Party I know,” Ian Lavery, a Corbyn supporter, told me. The continuation of “internal warfare”, he warned, would lead to “unmitigated disaster”. The more central the issues of foreign and defence policy become to British politics, the greater the potential for division. As they reflect on the chasm between themselves and their leader, it is the choice of a nuclear strike or of “prolonged guerilla fighting” that Corbyn’s foes now confront.