Keir Starmer’s defence investment plan leaves behind spending problems that his successor will not be able to avoid. Military budgets will be well short of the UK’s Nato commitments by the end of the decade, and European allies and a combustible White House are likely to notice. Yet finding further cash for the profligate Ministry of Defence will require squeezing other departments, because raising taxes would be fraught and the headroom for extra borrowing is limited.

Another option is to abandon Britain’s residual ambitions to be more than a regional power, though it is not one allies are keen for the UK to take. The complication is that the investment plan is trying to tackle several major problems at once. The post cold war peace ended more than four years ago with Russia’s invasion of Ukraine and, as the plan document drily notes, “the world has changed”, with the threat from Moscow heightened and the Middle East volatile.

At the same time, Labour ministers are right to say their Conservative predecessors left behind a funding mess, while increasing commitments by extending support to Ukraine and signing up to develop nuclear-powered submarines with Australia and the US, and fighter jets with Japan and Italy. Dan Jarvis, the new defence secretary, said on Tuesday 47 out of 49 major projects were delayed or over budget. As the defence investment plan makes clear, at its heart is a massive investment in nuclear, which accounts for 20% of the entire defence budget and is rising to 25% as modernisation continues.

A total of £47bn is being spent on nuclear submarines, principally Dreadnought boats to replace the Trident-carrying Vanguards, now so old that their crews have to endure record-breaking patrols of six months plus. Yet, as the public accounts committee complained in June, the nuclear programme has been subject to limited scrutiny by parliament because it is considered too sensitive. The mystery, though, is why it took Labour so long to grip defence costs.

The reckoning came after the publication of a lightweight review last year and a full three-year spending review. It was only towards the end of last year that it emerged the MoD’s sprawling ambition was £28bn underfunded – prompting months of bitter ministerial wrangling. Meanwhile, Donald Trump arrived at the White House with fluctuating enthusiasm for Nato and a firm belief that European allies had been free-riding on US defence expenditure.

Starmer tried once to placate Trump by agreeing to slash overseas aid to increase defence spending to 2.5% of GDP by 2027, a limited rise that for a moment maintained UK leadership in Nato. The problem was that Trump wanted to go much further, to 3.5% by 2035, and Nato allies, desperate to keep the US president on board, fell into line at the Nato summit last summer. So did Starmer, though the easy spending decisions had already been made.

Amid a lack of headroom in the UK’s public finances, ministers fell out, leaving the final settlement at 2.7% of GDP by 2030, well short of the Nato target. Government sources said Rachel Reeves, the chancellor, found it frustrating to deal with John Healey, the former defence secretary, because of his refusal to negotiate directly with her, choosing instead to talk to Starmer.