John Healey has inflicted what may prove a fatal wound to the Prime Minister, and by so doing, positioned himself as a possible replacement for Keir Starmer.The former Defence Secretary had been considered the most loyal of Starmerites, a long-time friend of Sir Keir and a most unlikely of political assassins.On the morning of the most damaging departure of Starmer's premiership, Healey posted pictures on his social media.The Defence Secretary was all smiles in photos taken early on Wednesday when he went for a jog around central London with visiting Australian deputy prime minister Richard Marles. Officials from both countries were looking forward to a joint visit to a defence manufacturer in Gosport, Hampshire, to underline cooperation between the UK and Australia as part of the AUKUS submarine alliance.There was no indication then of the drama soon to unfold. Hours later, some of his Antipodean guests found themselves stranded on the south coast as Mr Healey quit and the event at Qinetiq was cancelled. Mr Marles promptly cancelled other speaking engagements to avoid being drawn into the UK defence spending crisis.Rachel Reeves, a conscientious objector regarding defence spending, had offered a paltry £10billion uplift to meet the UK's additional security requirements, a figure lower than the most pessimistic of predictions ahead of the publication of the long-overdue Defence Investment Plan (DIP). John Healey (pictured, right) had been considered the most loyal of Starmerites, a long-time friend of Sir Keir and a most unlikely of political assassins But on Thursday he dramatically quit, becoming the most damaging departure of Starmer's premiership While nobody in Whitehall thought the Treasury would offer the Ministry of Defence anywhere near the £28billion demanded by service chiefs, nobody thought Ms Reeves would contemplate a sum as low as £10billion – split over four years in real terms. For months a figure of £18billion had been mooted.As Healey implied in his resignation letter, he had not been party to the Chancellor's discussions with the Prime Minister, and only learned this morning that the weakened PM was unwilling to go to war with No 11. Mr Healey's disappointment was palpable, the settlement 'falling well short of what is required' to keep Britain safe. He may also have felt let down personally having repeatedly defended the Prime Minister's commitment to national security in the face of withering criticism from Donald Trump.A stalwart of Labour's frontbench, Mr Healey has held the defence brief since Sir Keir won the party leadership in 2020.Described by an ally as 'one of the most loyal Labour men for over 30 years', he served in the Blair-Brown governments as well as the shadow cabinets of Ed Miliband and Jeremy Corbyn. His three-decade-long parliamentary career has seen him back the Iraq invasion and criticise the Tory government's reduction in the size of the Army to 72,500 by 2025.And since the General Election, the 66-year-old MP for Rawmarsh and Conisbrough had brought a sense of calm and purpose to the MoD, ably continuing Britain's support for Ukraine. Mr Healey criticised Sir Keir's (pictured) defence investment plan, which he felt fell 'well short of what is required' to keep Britain safe Rachel Reeves (pictured) a conscientious objector regarding defence spending, had offered a paltry £10billion uplift to meet the UK's additional security requirementsPhlegmatic by nature, Healey at times appeared ponderous and his speeches lacked vigour. But his commitment to improving the UK's Armed Forces was never called into question. Throughout his tenure he remained on top of his brief and alert to the gathering storm of Kremlin expansionism.However, since the publication of the Strategic Defence Review last summer, the Government's failure to publish the DIP became a growing cause of frustration.On Thursday night, sources close to Andy Burnham's camp expressed fears Healey could stand as a leadership candidate. He would appeal to pro-Starmer factions within Labour and is more engaged in defence issues than Burnham.In the aftermath of Mr Healey's resignation, the previously prized role of Defence Secretary has become a political poisoned chalice. His replacement, Dan Jarvis, will be required to support a spending agreement he has already described as weakening the country's defences and putting our troops in danger.Healey was always regarded within Whitehall as a safe pair of hands. Few thought those hands would wield a blade that wounded the PM.Before Thursday, the prospect of him doing so was no less likely than the prospect on Friday of him emerging as a centrist candidate in a Labour leadership race.From a national security perspective, arguably the country could sleep more easily with Mr Healey in Downing Street than Burnham or Wes Streeting.