It has been one of the few political constants in a turbulent period for British politics: an agreement that defence spending really should increase. But in the second era of Donald Trump, what was a consensual background hum has suddenly become an ear-splitting alarm.

European Nato members, the UK among them, have long been used to US presidents urging them to spend 2% of GDP at a bare minimum, something only a minority of them nonetheless manage.

Now, with Trump seemingly more intent on deciding Ukraine’s future with Russia than with European allies, and his defence secretary, Pete Hegseth, warning that “Uncle Sucker” can no longer be fully relied on to defend Europe, this is beginning to look like complacency.

The UK currently spends fractionally more than 2.3% of its GDP on defence, well above the Nato average if some way below America’s 3.3%.

More pertinently in a debate which can become bogged down in talk of percentages, British military leaders believe the current annual sum of £64bn is not enough even for current military commitments, let alone mooted new ventures such as involvement in a post-peace multi-nation force to protect Ukraine.

The government has committed to increasing defence spending to 2.5%, although ministers have not yet gone past the Labour manifesto commitment to “set out the path” to this, with no timescale yet given.

In contrast, the Conservatives, with the electoral luxury of knowing they were likely to go into opposition, promised to hit this mark by 2030, without much in the way of explanation about how it would be done. This does, nonetheless, put more pressure on ministers.

Some reports have sought to present the situation as Keir Starmer trying to persuade a reluctant Rachel Reeves to loosen the purse strings, something dismissed as a nonsense by both Downing Street and the Treasury.

The official narrative is that it all depends on not just the upcoming cross-government spending review but also the strategic defence review commissioned after the election and headed by George Robertson, the former defence secretary and then Nato head.

There are perfectly good arguments – ones made regularly by ministers – that just chucking more money at defence without a plan for what you want to achieve can be wasteful and counterproductive, but however many reviews you hold, they all lead to the same fiscal impasse.

With growth sluggish, Reeves’s fiscal headroom fast vanishing, public services already stretched and further major tax rises ruled out, there is no obvious way to raise the annual £7bn-plus or so it would take to hit the 2.5% threshold. Under Joe Biden, this was a problem which could wait for something to turn up. Not now.

All sorts of back-of-the-envelope answers have been mooted by pundits and observers, including a tax rise sold to the public specifically on the basis of protecting Ukraine, or “Ukraine bonds”, new wartime-style public borrowing.

Neither option is beyond the bounds of reality, and other European Nato countries have already considered similar ideas. But for now, UK officials are insistent that nothing will be rushed. Jonathan Reynolds, the business secretary who was on government media duty on Sunday, made the point that, as well as just money, issues such as domestic defence manufacturing capabilities should be considered.

Such caution is arguably part of a wider phenomenon of UK politicians valiantly trying to argue that Trump’s America is the same dependable ally it always was and not the fickle, treaty-shredding transactional force it has suddenly become.

The last week has already seen that pretence hit the very solid wall of a new reality. Everything is changing, at speed, and defence spending looks like it might just have to catch up.