Yet in many ways this inversion of Spengler is a fundamental misreading of the trajectory of the last hundred years. Far from being a time of Western ascendancy, the past century has in reality witnessed something more like a re-orientation of the world—albeit only a partial re-orientation—and the relative decline of the West.

In 1900 the West really did rule the world. From the Bosporus to the Bering Strait, from Siberia to Ceylon, nearly all of what was then known as the Orient was under some form of Western imperial rule. The British had long ruled India, the Dutch the East Indies, and the French Indochina; the Americans had just seized the Philippines; the Russians aspired to control Manchuria. All the imperial powers had established parasitical outposts in China. The East, in short, had been subjugated, even if that process involved far more complex negotiations and compromises between rulers and ruled than used to be acknowledged.

Western hegemony was one of the great asymmetries of world history. Taken together, the metropoles of all the Western empires—the American, Belgian, British, Dutch, French, German, Italian, Portuguese, and Spanish—accounted for 7 percent of the world’s land surface and just 18 percent of its population. Their possessions, however, amounted to 37 percent of global territory and 28 percent of mankind. And if we regard the Russian empire as effectively another European empire extending into Asia, the total share of these Western empires rises to more than half the world’s area and population. This was a political globalization unseen before or since.

What enabled the minority in the West to rule the majority in the East in 1900 was not so much scientific knowledge in its own right as its systematic application to both production and destruction. By contrast, the empires of the East, from the Ottoman to the Qing, failed disastrously to modernize themselves. Their economies remained trapped in subsistence agriculture while the West forged ahead, colonizing and industrializing, devouring sugar and burning coal. Their tax systems were inefficient, forcing Oriental rulers to borrow from Western capital bankers. Eastern armies remained long on pageantry and short on firepower, while the West could deploy well-drilled troops equipped with machine guns and heavy artillery. Eastern navies stood no chance against the Western combination of steam and steel.

Nothing symbolized better the humiliation of the East than the Western military intervention to suppress the Boxer Rebellion, in China, in 1900. The rebels, who had menaced Western diplomats and missionaries, relied on martial arts and magic. Having wiped them out, the Western expeditionary force staged a “grand march” through Beijing’s Forbidden City and then undertook punitive raids deep into Shanxi Province, Inner Mongolia, and Manchuria.

Just a few years later, however, the East began to re-assert itself. Japan’s defeat of Russia on land and at sea in 1904–5 marked a turning point in world history. From that point on, the balance of geopolitical power began to turn, slowly and painfully, back toward the more populous part of the world. It is only when the extent of Western dominance in 1900 is appreciated that the true narrative arc of the 20th century reveals itself. This was not “the triumph of the West,” but rather the crisis of the European empires, the ultimate result of which was the revival of the East—beginning in Japan—and the relative decline of the West.

This has not been a decline in the sense that Spengler envisaged: a kind of corrosive metropolitan ennui. Rather, it has been an unexpected but inexorable military decline. It has been a scarcely perceptible economic decline. It has been a subtle but unmistakable cultural decline. Above all, it has been a creeping demographic decline. In short, it has been a decline in precisely the sense that Gibbon understood the decline of Rome’s empire.