This is John Healey’s finest hour

Either we have to spend less on defence, and say what we will not be able to do, or we have to meet the target Sir Keir Starmer himself set, and say how we would pay for it. Either course makes sense. It could be argued that the prime minister allowed himself to be bounced into setting a target of spending 3 per cent of national income on defence by Donald Trump and the British defence establishment, according to The Independent.

As John Healey’s resignation shows, many of his former cabinet colleagues are secretly of this view. They do not think, when asked to make cuts to their own departments, that the 3 per cent target really should be delivered by the end of the decade. Rachel Reeves, the chancellor, when asked to choose between softening the target or raising taxes, decided that 2.68 per cent was nearly as good as 3 per cent.

But there are good arguments for spending 3 per cent. The drawing down of the United States’ contribution to the defence of Europe at a time when Vladimir Putin is in his fifth year of trying to subjugate Ukraine, and the instability in the Middle East, together make the case. The Independent supported the prime minister when he set the 3 per cent figure as his ambition for 2030. We have argued that savings could be made to help pay for it, including restraining the growth in disability benefits and adjusting the triple lock on uprating the state pension. But if necessary, we would accept an increase in income tax.

What does not make sense, however, is saying that we need to spend 3 per cent and then backing off when it proves too politically difficult.

Mr Healey should be congratulated not just for resigning on an issue of principle, but for forcing the nation to face the truth. His letter of resignation is absolutely unforgiving, setting out what Sir Keir agreed and the way in which he then reneged on it. He says that since last year’s Strategic Defence Review, the prime minister has been “unable”, and the Treasury has been “unwilling”, to “commit the resources that the nation needs to defend the country at this time of rising threats”.

That distinction between “unable” and “unwilling” is pointed and unflattering to Sir Keir. It is Ms Reeves’s job as chancellor to control public spending, and part of that involves being highly sceptical about the value for money provided by the Ministry of Defence.

But it is Sir Keir’s job to set the priorities for his government and to overrule the Treasury if necessary. Mr Healey, in effect, charges the prime minister with being too weak to insist that when he said 3 per cent, he meant it.

It is a damning charge. “Without a Defence Investment Plan that meets the moment in this way, I am being forced to make decisions that would reduce the readiness of our forces and increase the risk to personnel on operations, and could make the country less safe,” Mr Healey writes.

His resignation should put an end to Sir Keir’s hope of seeing off a challenge to his leadership, and is likely to mean two things. One is that Sir Keir will cease to be prime minister sooner rather than later, and the other is that his successor will set out plans to spend more on defence over the next four years.