It was under the Attlee government in 1949 that the UK co-founded Nato and became one of its senior members. Every Labour leader since has supported the military alliance. But in last night's hustings, Jeremy Corbyn refused to commit to upholding Article 5: the principle of collective defence ("an attack against one Ally is considered as an attack against all Allies").

When asked how the UK would respond if a fellow Nato member state was invaded by Russia, Corbyn replied: "I would want to avoid us getting involved militarily by building up the diplomatic relationships and also trying to not isolate any country in Europe, to bring them up." Pressed on the subject, he added: "I don’t wish to go to war. What I want to do is achieve a world where we don’t need to go to war, where there is no need for it. That can be done."

Corbyn's stance is not especially surprising. During last year's leadership election, he suggested that Nato should have been disbanded ("It's a Cold War organisation, it should have been wound up in 1990 along with the Warsaw Pact"), though he later retreated and stated that there wasn't "an appetite as a whole for people to leave". But his stance on Article 5, which echoes that of Donald Trump, has resurrected the divide.

Owen Smith, his rival candidate, replied: "Were there an invasion of a Nato state by Russia, I am clear that we would need to come to the aid of that state militarily. I believe in us supporting one another, I believe in us working against countries. And the nature of that accord, that treaty, is to do that." He added: "We shouldn't be anything other than robust in facing up to Putin, especially if there were military action against a Nato country." Wes Streeting MP has described Corbyn's stance as "a gross betrayal of Labour's internationalist values".

As I noted earlier, no other party leader has ever adopted this position. Though it is often claimed that Labour's 1983 manifesto pledged Nato withdrawal, it actually stated: "Labour believes in collective security. The next Labour government will maintain its support for Nato". It is for reasons such as these that 172 Labour MPs voted no confidence in their leader last month and 65 resigned from his frontbench. Corbyn will now find it even harder to persuade any to return in the likely event of his re-election.