Iain Duncan Smith | Carl Court/Getty Images Westminster Sketch Duncan Smith plunges the Tories into civil war The UK welfare secretary’s resignation is a direct assault on George Osborne.

On the face of it, there is very little about Iain Duncan Smith’s resignation that makes sense. The long-serving welfare secretary stormed out of government late Friday evening, in protest at the implementation of a policy that was already being abandoned and which his own department had already approved. Yet his departure — on the most acrimonious possible terms — has plunged a Conservative Party already reeling over Europe into a fresh round of dispute and division.

Some people have linked Duncan Smith’s departure to the Brexit referendum, given his lifelong Euroskepticism. But there is a much simpler explanation for it: he’d had enough of George Osborne.

During his brief and disastrous stint as Tory leader between 2001 and 2003, Duncan Smith — known universally as “IDS” — visited the Easterhouse estate in Glasgow, one of the most deprived in Britain, and was deeply moved by what he saw there. After being removed as leader, he devoted himself to this brand of compassionate conservatism, devising benefit reforms intended to skew incentives towards work and away from welfare.

Osborne and David Cameron both supported this mission. But Osborne also needed to balance the books — and he did so, time after time, on the back of the welfare budget. The incentives to work that IDS hoped to introduce were whittled away, billion by billion: policy became focused on the stick, rather than the carrot.

The older man, forced to work under his two posher, brainier juniors, felt himself not just sidelined but sneered at.

This was, unlike many cuts, electorally popular: many of the public saw welfare claimants as scroungers, not sufferers. But it provoked a series of running battles between the two ministers — to the point where threats from IDS to resign over the latest insupportable cuts, and concessions from Osborne, became almost a tradition.

The relationship was not helped by the struggles of IDS’s flagship program, Universal Credit. This was, essentially, an attempt to weld together the two biggest databases in government: the list of what people paid in tax and what they received in welfare. The idea was and remains a good one, but whatever his other strengths, IDS was not a project manager or IT expert. The program has dragged on far beyond its original completion date. At one point, the Cabinet Office essentially engaged in a hostile takeover to sort out the whole mess.

The rows weren’t just political, but personal. Osborne thought IDS was engaged in “a quasi-religious program of mass redemption” rather than a thought-through program of reform, and was ultimately “just not clever enough,” according to Matthew d’Ancona’s history of the 2010-2015 coalition, "In It Together."

The older man, forced to work under his two posher, brainier juniors, felt himself not just sidelined but sneered at.

The final straw for IDS appears to have been Osborne’s proposal, in the national budget this week, to cap Personal Independence Payments, the credits given to the disabled. Osborne argued the cost of these has been soaring, and that he planned not to cap them but to limit their rise. His critics accused him of saving £4.4 billion in five years by punishing the disabled: the plan had already provoked a backlash, which in turn led to a government retreat.

But not hastily enough. IDS had been expected to go quietly after the Brexit referendum; instead, he went explosively. His resignation letter was a grenade thrown not at the prime minister, to whom it was formally addressed, but his chancellor. He savaged the politically driven policy of protecting pensioners (who vote, and vote Tory) while punishing the working-age, and condemned the “salami-slicing” of the welfare-to-work program. “I am unable to watch passively,” he wrote, “while certain policies are enacted in order to meet the fiscal self-imposed restraints that I believe are more and more perceived as distinctly political rather than in the national economic interest.”

The most vicious lines, however, were saved till last: “I hope as the government goes forward you can look again, however, at the balance of the cuts you have insisted upon and wonder if enough has been done to ensure ‘we are all in this together’.”

It was Geoffrey Howe's resignation speech that brought

down Margaret Thatcher, even though being attacked by Howe had been compared to being "savaged by a dead sheep."

Osborne has been mauled by a mastiff he thought he had neutered long ago.

Even though they have been in charge for more than 10 years, Cameron and Osborne have never had a solid ideological majority among their backbenchers.

So what will be the consequences? Electorally, it is a significant setback for the Tories. Their greatest vulnerability has always been the charge that they are competent but callous, more concerned about their rich friends than the struggling poor. IDS’s letter effectively provides proof of that: it explicitly singles out Osborne’s decision to cut capital gains tax and raise income tax thresholds for the rich while clamping down on payments to the disabled as morally insupportable. Meanwhile, it bolsters Jeremy Corbyn’s position: it will be hard for Labour moderates to argue for his removal when even a Tory Cabinet member appears to share his diagnosis of the government’s flaws.

It also changes the picture within the Tory party. Even though they have been in charge for more than 10 years, Cameron and Osborne have never had a solid ideological majority among their backbenchers. Support for their agenda depended as much on a recognition of the need to remain united to win elections; of Cameron’s own authority; and of Osborne’s ability to make life hell for those who crossed him.

Wise MPs still fear and respect the chancellor, but not as much as they did even a month ago. And this process is self-reinforcing. The less they believe Osborne is likely to succeed Cameron, the more comfortable his fellow Tories will feel in taking him on. MPs on the other side of the Brexit battle already felt slighted by the leadership’s hardball tactics, and justified in extending their opposition to other areas. Extending retail hours on Sundays, the disability cuts, changes to pension rules — on issue after issue, Osborne has received a bloody nose from his backbenchers recently, or been threatened with one. IDS’s resignation will surely pour rocket fuel on that fire.

It will also change the dynamic within government. The policy agenda across Whitehall, for the past six years, has been driven by a Treasury every bit as imperial as in Gordon Brown’s day. If Osborne becomes weaker, departments may well rediscover their ability to say no.

Finally, it will inevitably throw the outcome of the Brexit referendum into further doubt. With such a small constituency of genuine Europhiles in the country, the Remain side was always going to rely heavily on the personal influence of the prime minister, the father of his country, who had weighed the options and made the most judicious choice.

But that depended on Cameron appearing a figure of stability, leading a government that was competent, seasoned and effective. An administration beset by internal squabbling, engaging in a forced reshuffle, whose divisions over Europe have already seen it slump in the polls, makes for a rather less impressive platform. Even Cameron and Osborne’s inevitable, and necessary, counter-attack on IDS will fuel the impression of a Tory civil war. Resignation will also free Duncan Smith to campaign for Leave with extra vigor, and fewer inhibitions.

There is, for Tory historians, a certain irony here. IDS made his name in the 1990s as one of the irreducible core of Euroskeptics whose opposition fatally weakened John Major’s premiership. His latest vendetta against his party’s leadership may have even greater consequences.

This article was updated to correct a reference to Geoffrey Howe's resignation speech.