Despite the bitter war they fought in the 1940s, Japan and Britain (my native country) have much in common. Both are archipelagos off the vast Eurasian landmass. Both are among the most densely populated countries in the world. Both were once mighty empires. Both are still quite rich. Both are constitutional monarchies.

Yet while Britain today is in a state of acute political crisis, Japan seems a model of political stability. Is this a matter of personalities — the sad fact that Theresa May is a talentless leader, Shinzo Abe a gifted one? Partly. But there is more to it than that.

The Japanese, crushed in 1945, conceded only a superficial Americanization of their culture and institutions. To a remarkable extent, Japan did not change. It merely jettisoned the hysterical nationalism that had come to the fore in the 1930s. Not only did the Emperor survive, but so did the country's social elite. They accepted land reform but retained political power.

The continuities of Japanese history are exemplified by Prime Minister Abe's political pedigree. His great-great-grandfather was a general in the Imperial Japanese Army. His grandfather on his mother's side was a member of Hideki Tojo's Cabinet during World War II and prime minister in the late 1950s. His other grandfather was a member of Parliament (and an opponent of Tojo). Abe's father was Japan's foreign minister in the 1980s.

But the continuities also manifest themselves in the complex system of manners that governs Japanese social life. Nowhere in the world will you encounter such politeness. After two days in Tokyo last week, my back hurt from bowing, and I had said "Arigatou gozaimasu" (thank you very much) at least a thousand times.

The contrast with Britain's postwar history is striking. Victorious in war, we jettisoned as much as we could of our Victorian and Edwardian heritage. A new class entered politics from state schools and red-brick universities, greatly diluting the hereditary element in Parliament.

As for our manners, which were once famously straitlaced, there has been a precipitous decline into vulgarity, so that Americans now seem polite by comparison. Last week, according to a BBC journalist, a cabinet minister responded with an expletive when asked why Theresa May was holding yet another Brexit vote. Even members of the elite now talk like louts.

In many ways, you might think, Japan has much bigger problems than Britain. According to the World Bank, the old-age dependency ratio in Japan is 45 percent, the highest in the world. Britain, by contrast, ranks 17th in the senescence league table. Japan's gross public debt is now 248 percent of gross domestic product, again the world's highest. Britain's is 87 percent, according to the International Monetary Fund, putting us in 29th place. Britain leads Japan in terms of innovation, economic and political freedom, ease of doing business, and even happiness.

And yet consider the political states of the two countries. In Japan the main question is whether Abe — who has been prime minister since 2012 and who has led his Liberal Democratic Party to three successive election victories — should stay on beyond 2021, when his term as LDP leader is supposed to end. The other big question (to be resolved Monday) is what the name of the new imperial era will be when the Emperor Akihito abdicates to make way for his son Naruhito.

Meanwhile, in London, a political crisis of 17th-century magnitude continues to unfold. Three years ago, I was writing articles on these pages, warning that it would be much harder for Britain to leave the European Union than the proponents of Brexit were claiming. I underestimated the degree of difficulty. I did not imagine that the Brexiteers themselves would end up voting against Brexit, as a significant number of them did for the third time on Friday.

So badly has Theresa May bungled Britain's great divorce that she could not even get her Withdrawal Agreement passed by promising to resign if MPs voted for it. The result is that Britain's political fate now depends on . . . the European Union, which gets to decide whether or not to grant Britain a longer extension than the 11 days currently remaining before we leave the EU without a deal and enter an economic crisis of unknown scale and duration.

Why are Japan and Britain in such different political states? The superficial answer is that there was never an Asian Economic Community that Japan chose to join in the 1970s. Across the water, there is just the vast, authoritarian superpower that is China.

A more profound answer is that, while Britain has embraced immigration, Japan has resisted it. True, there has been a quiet uptick in the number of foreigners residing in Japan since 2014. There are now more than 1.2 million foreign workers in the country, the majority from other Asian countries. But the foreign-born share of the population is just 2 percent, according to the World Bank. The equivalent figure for Britain is 13 percent.

Future historians will wonder why Britain's Conservatives decided to commit seppuku over Brexit. But perhaps conservatism itself is just incompatible with immigration on this scale, and the Brexit breakdown is merely a symptom. The Japanese probably think that — but they are too polite to say it.

Niall Ferguson is the Milbank Family senior fellow at the Hoover Institution, Stanford University.