Defence row exposes tensions over how to keep UK safe

But successive governments have struggled to get a grip on defence spending. They spent less after the Cold War ended and failed to spend more as the world became more dangerous. As a result, the army, navy and air force contracted.

To its many critics, the Ministry of Defence failed to spend well what money it got, botching procurement after procurement, delivering new equipment late and over budget. Whitehall often failed to resolve its internal tensions, both No 10 and Cabinet Office unable – or too weak – to manage repeated rows between Treasury and MoD.

What is different about this row is its context. The world is changing fast. The nature of the threat against the UK is changing. Less immediate concern about non-state actors and militant groups such as al-Qaeda or ISIS. More focus instead on state-on-state aggression such as from Russia and Iran.

That threat is increasing. In his letter replying to John Healey’s resignation, the prime minister wrote: “The world today is more dangerous and uncertain than at any point in our lifetimes”.

The nature of warfare is also changing. Less need, perhaps, for traditional armour and ships. More focus on drones, cyber, space and technology as the wars in Ukraine and the Gulf have shown. What matters is the mass production of cheap and lethal projectiles that are adapted in real time, not long-term investment in big and vulnerable platforms that become outdated fast.

And all this is happening at a time when the United States has made it clear that it is no longer prepared to subsidise the defence of its European allies, demanding they finally meet their Nato spending targets.

These factors have made the need to spend more on defence more pressing. In response, the government made bold promises. The UK spent 2.3% of gross domestic product – or national output – on defence last year. Labour says that will rise to 2.5% by next year. And then, it promised to increase spending further, along with other Nato allies, to 3.5% of GDP by 2035. At the Munich security conference in February, the prime minister pledged: “We are going to have to spend more faster”.