John Healey resigned as defence secretary following continued disagreement between Downing Street, the Treasury and the Ministry of Defence over the defence investment plan. Healey said the plan falls “well short of what is required for defence and the country at this dangerous time”.His departure on June 11 was followed by Al Carns’ resignation as armed forces minister. Carns said in a letter to Keir Starmer that the funding plan is “not built for the threat we face”.The resignations come at a time of political turmoil for the prime minister, following the huge losses Labour faced in the local elections last month, the subsequent resignation of Wes Streeting as health secretary and the threat posed by Greater Manchester Mayor Andy Burnham’s potential arrival back in Westminster. What is the defence investment plan?The row is centred around funding for defence spending. Following the 2024 general election, Starmer commissioned a strategic defence review to set out a vision for UK defence over the next ten years and ensure the military is prepared for present and future threats. It was explicit in setting out the future mix of our armed forces, as well as the number of aircraft, ships and vehicles the military would need. What it didn’t do, however, was provide insight into how it was to be funded. This is where the defence investment plan comes in. It is expected to explain how the new equipment and infrastructure detailed in the defence review will be paid for. The defence review was published a year ago, so we have been waiting to find out how this would be funded for some time. Read more: The strategic defence review means three new approaches for the UK Healey said that he had “no other option” but to quit after the prime minister was “unable” and the Treasury was “unwilling” to commit to the resources that Healey believes are needed to be able to defend the UK effectively. “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,” he wrote to the prime minister.It has been reported that the Treasury was refusing to offer more than £13.5 billion in investment (a 0.08% budget increase) when Healey and the Ministry of Defence had asked for £18 billion. Starmer responded to Healey’s letter saying that the funding in defence investment plan as it stands would “transform and modernise” the armed forces, while being “sustainable and fair”.To give the Treasury some credit, the Ministry of Defence is known for poor spending decisions when it comes to procuring equipment. It has long been criticised for wasting taxpayers’ money. So their hesitancy may be justified at a time when the public finances are very tight and the cost of government borrowing has risen. In order to pay for the full £18 billion package that Healey wanted, spending cuts in other departments would have been required. Indeed, a Treasury source’s reported response to the critiques in the resignation letter point to the need to cut the NHS and schools budgets to find the necessary money – something the Treasury and the prime minister are unwilling to do, given the promise to end austerity. How bad is it for Starmer?This critique from Healey is particularly damaging for Starmer. It implies that he lacks both the political capital and the authority to make such a decision. This further reinforces expectations that he may not remain in office for much longer, particularly if Burnham wins the Makersfield byelection later this month and challenges Starmer for the party leadership.In light of this, Starmer had intended to demonstrate that he was getting on with the job. The defence investment plan was supposed to form part of that political fightback in the hope of retaining office. While Starmer has also been more widely praised for his work on the world stage, this row, and the failure to push the defence review and investment plan through, has undermined that perception.This is not the first public dispute over the defence investment plan. Back in April, Lord Robertson (a Labour peer, former defence secretary, former Nato secretary general, and author of the strategic defence review) launched a scathing attack on the government for its failure to implement and properly fund the review. Read more: The UK is alarmingly unprepared for the threats it faces – security expert explains why Healey is widely seen as a Starmer loyalist, and his resignation has come as a shock to Labour MPs. Starmer’s authority has clearly been weakened over the past few months, and the resignations further undermine his position. While Starmer has maintained that he would contest any leadership challenge (under Labour rules he would be an automatic candidate) this week reduces his chances of securing victory in such a contest.It also damages Starmer on the international stage. He is due to attend the G7 meeting in France next week, where he will meet Donald Trump for the first time since their falling out over the conflict with Iran. He is likely to face criticism from both the US president and potentially other G7 allies over the failure to secure a funded defence plan.There is also an upcoming Nato summit in July, and there is a risk that the UK could lose its status as a leading ally within the alliance, as other European countries rearm and reach defence spending targets at a much faster pace.The appointment of Dan Jarvis (previously the security minister and a former British Army officer) as defence secretary nearly nine hours after Healey’s resignation is a sound choice. The question now is whether the length of time taken to appoint him reflects the need for additional security checks for the role, or whether he was negotiating with Downing Street over the details of the spending package.On the horizon, too, is the possibility of a leadership contest, a new prime minister, and a subsequent change of government. As a result, it is entirely plausible that Jarvis may hold the position for only a matter of months.Thomas Caygill has previously received funding from the British Academy/Leverhulme Trust and the Economic and Social Research Council.