In December, as many as 500 patients per week were dying in Britain because of E.R. waits, according to the Royal College of Emergency Medicine, a figure rivaling (and perhaps surpassing) the death toll from Covid-19. On average, English ambulances were taking an hour and a half to respond to stroke and heart-attack calls, compared with a target time of 18 minutes; nationwide, 10 times as many patients spent more than four hours waiting in emergency rooms as did in 2011. The waiting list for scheduled treatments recently passed seven million — more than 10 percent of the country — prompting nurses to strike. The National Health Service has been in crisis for years, but over the holidays, as wait times spiked, the crisis moved to the very center of a narrative of national decline.Post-Covid, the geopolitical order has been thrown into tumult. At the beginning of the pandemic, commentators wondered about the fate of the United States, its indifferent political leadership and its apparently diminished “state capacity.” Lately, they have focused more on the sudden weakness of China: its population in decline, its economy struggling more than it has in decades, its “zero Covid” reversal a sign of both political weakness and political overreach, depending on whom you ask.But the descent of Britain is in many ways more dramatic. By the end of next year, the average British family will be less well off than the average Slovenian one, according to a recent analysis by John Burn-Murdoch at The Financial Times; by the end of this decade, the average British family will have a lower standard of living than the average Polish one.On the campaign trail and in office, promising a new prosperity, Boris Johnson used to talk incessantly about “leveling up.” But the last dozen years of uninterrupted Tory rule have produced, in economic terms, something much more like a national flatlining. In a 2020 academic analysis by Nicholas Crafts and Terence C. Mills, recently publicized by the economic historian Adam Tooze, the two economists asked whether the ongoing slowdown in British productivity was unprecedented. Their answer: not quite, but that it was certainly the worst in the last 250 years, since the very beginning of the Industrial Revolution. Which is to say: To find a fitting analogue to the British economic experience of the last decade, you have to reach back to a time before the arrival of any significant growth at all, to a period governed much more by Malthusianism, subsistence-level poverty and a nearly flat economic future. By all accounts, things have gotten worse since their paper was published. According to “Stagnation Nation,” a recent report by a think tank, there are eight million young Brits in the work force today who have not experienced sustained wage growth at all.Over the past several decades, the China boom and then the world’s populist turn have upended one of the basic promises of post-Cold War geopolitics: that free trade would not just bring predictable prosperity but also draw countries into closer political consensus around something like Anglo-American market liberalism. The experience of Britain over the same period suggests another fly in the end-of-history ointment, undermining a separate supposition of that era, which lives on in zombie form in ours: that convergence meant that rich and well-governed countries would stay that way.For a few weeks last fall, as Liz Truss failed to survive longer as head of government than the shelf life of a head of lettuce, I found myself wondering how a country that had long seen itself — and to some significant degree been seen by the rest of the world — as a very beacon of good governance had become so seemingly ungovernable. It was of course not that long ago that American liberals looked with envy at the British system — admiring the speed of national elections, and the way that new governing coalitions always seemed able to get things done.Post-Brexit, both the outlook for Britain and the quality of its politics look very different, as everyone knows. But focusing on a single “Leave” vote risks confusing that one abrupt outburst of xenophobic populism with what in fact is a long-term story of manufactured decline. As Burn-Murdoch demonstrates in another in his series of data-rich analyses of the British plight, the country’s obvious struggles have a very obvious central cause: austerity. In the aftermath of the 2008 global financial crisis, and in the name of rebalancing budgets, the Tory-led government set about cutting annual public spending, as a proportion of G.D.P., to 39 percent from 46 percent. The cuts were far larger and more consistent than nearly all of Britain’s peer countries managed to enact; spending on new physical and digital health infrastructure, for instance, fell by half over the decade. In the United States, political reversals and partisan hypocrisy put a check on deep austerity; in Britain, the party making the cuts has stayed steadily in power for 12 years.The consequences have been remarkable: a very different Britain from the one that reached the turn of the millennium as Tony Blair’s “Cool Britannia.” Real wages have actually declined, on average, over the last 15 years, making America’s wage stagnation over the same period seem appealing by comparison. As the political economist William Davies has written, the private sector is also behaving shortsightedly, skimping on long-term investments and extracting profits from financial speculation instead: “To put it bluntly, Britain’s capitalist class has effectively given up on the future.” Even the right-wing Daily Telegraph is now lamenting that England is “becoming a poor country.”Of course, trends aside, in absolute terms Britain remains a wealthy place: the sixth-largest economy in the world, though its G.D.P. is now smaller than that of India, its former colony. And while the deluded promises of Brexit boosters obviously haven’t come to pass, neither have the bleakest projections: food shortages, crippling labor crunches or economic chaos.Instead, there has been a slow, sighing decay — one that makes contemporary Britain a revealing case study in the way we talk and think about the fates of nations and the shape of contemporary history. Optimists like to point to global graphs of long-term progress, but if the political experience of the last decade has taught us anything, it is that whether the world as a whole is richer than it was 50 years ago matters much less to the people on it today than who got those gains, and how they compare with expectations. Worldwide child mortality statistics are indeed encouraging, as are measures of global poverty. But it’s cold comfort to point out to an American despairing over Covid-era life expectancy declines that, in fact, a child born today can still expect to live longer than one born in 1995, for instance, or to tell a Brit worrying over his or her economic prospects that added prosperity is likely to come eventually — at the same level enjoyed by economies in the former Eastern Bloc.Can Britain even stomach such a comparison? The wealthy West has long regarded development as a race that has already and definitively been won, with suspense remaining primarily about how quickly and how fully the rest of the world might catch up. Rich countries could stumble, the triumphalist narrative went, but even the worst-case scenarios would look something like Japan — a rich country that stalled out and stubbornly stopped growing. But Japan is an economic utopia compared with Argentina, among the richest countries of the world a century ago, or Italy, which has tripped its way into instability over the last few decades. Britain has long since formally relinquished its dreams of world domination, but the implied bargain of imperial retreat was something like a tenured chair at the table of global elders. As it turns out, things can fall apart in the metropole too. Over two centuries, a tiny island nation made itself an empire and a capitalist fable, essentially inventing economic growth and then, powered by it, swallowing half the world. Over just two decades now, it has remade itself as a cautionary tale.David Wallace-Wells (@dwallacewells), a writer for Opinion and a columnist for The New York Times Magazine, is the author of “The Uninhabitable Earth.” More