What on earth is Jeremy Corbyn on about? When asked at last night’s leadership debate in Solihull whether as prime minister he would aid a Nato ally under attack, he said no. “I would want to avoid us getting involved militarily. I want to achieve a world in which we don’t need to go to war.” The implication in this is that Corbyn wants to withdraw Britain from Nato.

The sole rationale of the alliance is – or should be - to come to the military assistance of any fellow nation under attack. It is a real deterrent, and its plausibility rests on the assurance of collective response. Vacuities about a better world are one thing. We all want that, but Nato is a military alliance. Its governors must keep their word.

This glimpse of Corbyn’s state of mind recalls Donald Trump’s similar jibe at “allies who don’t pay their way”. America is the master power in the Nato alliance. Any doubt as to its constancy is disastrous to the organisation’s credibility. But the same to a lesser degree applies to Britain, even to a British opposition party. An alliance is all for one and one for all.

This is quite separate from the issue of nuclear weapons. When Corbyn was thought to oppose a British nuclear deterrent I cheered. A potential prime minister was joining most thoughtful defence analysts in deploring the retention of these archaic and unusable weapons. Maintaining a nuclear deterrent is obscene and wildly extravagant. Nuclear bombs are toys for boys and lolly for lobbyists. A sensible opposition should ask instead what sort of defence Britain really needs in the 21st century.

In the event, Corbyn funked it. He conceded to Labour’s “parliamentary” wing, fearful of being chided in the Commons as soft on defence. Faced with an open goal, he shot wide. Now he appears to have lurched instead against Nato. That is a different matter. It has nothing to do with nuclear deterrence, unless Corbyn’s policy is to abandon Nato and rely instead on Britain’s nuclear force to defend his country. If so, the Kremlin must be weeping with joy.

Corbyn could sensibly have questioned Nato’s current purpose, not least its inexcusable support for America’s retaliatory war in Afghanistan. He could have warned against giving defensive assurances to non-Nato states along Russia’s borders. He could even have questioned the whole purpose of an alliance forged in the cold war, perhaps one that is no longer fit for purpose.

He did none of this. He implied that Britain’s central alliance is not an alliance, that Britain’s word was not its bond. That is wild.