The surprise thing about John Healey's resignation is that he never looked like a quitter.
He has always been a loyal minister and Labour Party man.
Unlike other recent defence secretaries, he studiously avoided briefing against other ministers or the Treasury.
When he met journalists, he - frustratingly for us - hid his own frustrations and mostly said the same in private as he did in public.
But the government's failure to move as fast as other European nations, or as fast as he had promised, on ramping up defence spending had clearly taken its toll.
Allies were asking when the UK government's words would be matched by actions, as was the defence industry.
Healey made mistakes too. He thought the government's promise to increase defence spending to 2.5% of GDP earlier than planned would be enough, and senior defence officials were exasperated when they heard he had said he would not be asking for any more.
He soon had to.
Healey was also accused of cutting out other ministers in his department from key conversations and decisions.
But all that being said, his resignation means the government has lost a hard-working, dedicated and - until now - loyal minister.
The BBC understands that behind the scenes John Healey had been pressing the prime minister for a larger increase in defence spending than he was offered.
The deal the prime minister put to him on Monday did not include a timetable to increase defence spending to 3% of GDP, an ambition the government has committed itself to, not least because of pressure from the Trump administration, which has tired of doing the heavy lifting on investment.
Healey was pushing the prime minister to commit to hitting that mark by 2030.
Having promised to reverse what he called the "hollowing out" of the armed forces, the settlement Healey was offered on Monday was not enough for him.
As he set out in his resignation letter, according to his analysis the increase on the table would have actually meant cuts to investment because of increasing demands on the military, not least supporting Ukraine, protecting Gulf allies from Iranian attacks and ongoing efforts designed to deter Russian aggression in the Arctic.
Healey was offered an extra £13.5bn over the next four years - which defence sources said would really mean a £10bn increase once "Treasury trickery" had been taken into account.
The offer was well short of what Healey had been asking for - and well below the estimated £28bn needed to avoid large cuts.
It is understood that Healey told the prime minister that what amounted to a percentage increase of 0.08% would not be enough to keep the country safe. The plan he was presented with took spending from 2.6% of GDP now to 2.68% in 2030.
That, ultimately, is why he walked.
While there has been praise for Healey's decision to resign as defence secretary, there has been criticism too.
Some who worked with him in the Ministry Of Defence said he had long been warned its budget was already overcommitted, stretching back to the previous government when he served as shadow defence secretary.
When he commissioned Labour's strategic defence review, it increased those unfunded ambitions "substantially".
The "hard part", a defence source said, was always going to be getting agreement on the defence investment plan - in other words, the extra money.
A source said that Healey still believed he could somehow balance the books, that big adjustments could be made - an assumption they described as "inept".
But at a time of international instability and with the government's defence plans still seemingly up in the air, Healey's resignation could hardly be more serious.
The politics are clear: this makes the prime minister look weaker.
But there is a big outstanding question we do not yet know the answer to: By resigning, has Healey strengthened the hand of the armed forces in his last act as defence secretary?
