The row within the country's main opposition party on whether to bomb Syria has deepened splits over Jeremy Corbyn's leadership, while reviving uncomfortable memories over the wars in Iraq and Afghanistan.
Prime Minister David Cameron is expected to call a vote this week on joining the international coalition targeting Islamic State extremists, but needs support from some Labour MPs to secure a majority.
The problem is that Corbyn — a left-winger who helped found the prominent Stop The War Coalition protest movement — opposes air strikes, while many of his MPs support the move.
As well as complicating Cameron's calculations on whether he can win the vote, the situation has unleashed a round of very public feuding between pro and anti-Corbyn MPs about whether he is the right man to lead the main opposition party.
It is no coincidence that the row is about military action, the most sensitive issue for Labour since former leader Tony Blair led Britain into supporting the US in Iraq on evidence that was later hotly disputed.
The wars in Iraq and Afghanistan are now deeply unpopular in Britain, which lost over 600 troops. "The Iraq factor is massive — it's front and centre" for Labour MPs deciding whether to support action in Syria, according to Victoria Honeyman, an expert on British foreign policy at Leeds University.
"They're worried that they will look like they're following the Blairite approach to war, which is tainted," she added.
The key question facing Corbyn is whether he will let Labour MPs vote with their consciences or impose a "three-line whip" — a way to try and force the whole party to vote against air strikes, with consequences for those who do not.
This is expected to be decided at a meeting on Monday, which promised to be explosive. Analysts say that offering a free vote to appease the rebels could leave Corbyn looking like he cannot control his own party.
Ahead of the meeting, the Labour Party said it had received tens of thousands of responses to a call by Corbyn last week for members to say which way they think the party should vote.
An initial analysis of 1,900 responses showed 75 per cent opposed to bombing, 13 per cent in favour and 11 per cent undecided, party officials said.
This survey has sparked fury among some, who accuse Corbyn of underhanded tactics by rallying support for his own position instead of waiting for the party to make a collective decision.
"How does Jeremy Corbyn and his small group of tiny Trots in the bunker think they've got the unique view on it all?" senior Labour MP John Spellar fumed on BBC radio Friday.
Another Labour lawmaker, Paul Flynn, told the BBC there were "terrible divisions" while a third, Fiona Mactaggart, called Corbyn's position "unsustainable".
But one of his closest allies, Diane Abbott, on Monday urged Corbyn to "show leadership" and impose his line on the party. She said that "the party as a whole" was opposed to the bombing.
Key to the row is the fact that Corbyn — a political outsider and serial rebel before becoming leader — was elected in September thanks to grassroots left-wing support, but is not widely backed by generally more centrist Labour MPs.
If Corbyn does impose a three-line whip and try to make Labour MPs vote against air strikes, Honeyman predicted that some of his frontbench team could quit. "If you're a member of the shadow cabinet and you defy a three-line whip, there's an expectation that your resignation will be on the desk in the morning," she said.
While that would be deeply embarrassing for Corbyn, it is not clear it would herald the end of his leadership. The process for removing a Labour leader is complex and Corbyn has a huge mandate from activists, making any challenge to him problematic. "I'm not going anywhere," the leader said on Sunday.
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