3rd century AD

Every settled arrangement the previous two centuries built comes apart within a few decades of each other across Eurasia, and almost none of the people living through it could have known how widely shared the experience was. Rome's Crisis of the Third Century, roughly 235 to 284, cycles through dozens of emperors and rival claimants, most killed by their own troops or a rival's, the empire briefly splitting into a breakaway Gallic empire in the northwest and a Palmyrene empire in the east before Aurelian forces reunification by 274. The Plague of Cyprian, an unidentified pandemic recurring from 249 to 262, kills at a rate contemporaries describe as emptying whole cities, striking an empire already fighting Germanic incursions on the Rhine and Danube and a newly aggressive Persia on its eastern frontier at the same time. Aurelian's response, a new defensive wall around Rome itself completed by 275, is a physical admission that the city's old security, unquestioned for eight hundred years, no longer holds.

That eastern pressure has a specific new source. Ardashir I overthrows the last Parthian king in 224 and founds the Sasanian dynasty, which proves immediately more centralized and more aggressive toward Rome than the looser Parthian confederation it replaces, fighting repeated wars along the Mesopotamian frontier and, in 260, capturing a Roman emperor, Valerian, alive in battle, a humiliation without precedent in Rome's previous three centuries of dealing with its eastern neighbors.

China's crisis runs on its own clock but lands in the same decades. The Han Empire formally ends in 220, fracturing into the rival kingdoms of Wei, Shu, and Wu, whose wars, including the Battle of Red Cliffs in 208, supply the material for centuries of later Chinese storytelling. Chen Shou, who lived through the period as an official under first Shu and then the reunifying Jin, compiles the Sanguo Zhi, an official history of the very collapse and reunification he had personally witnessed, completed only after the Jin dynasty restores a single Chinese state by 280, itself a reunification no more secure than Rome's own recent stabilization under Diocletian's four-way Tetrarchy of 293.

What is strangest about this century, in retrospect, is how much serious intellectual work continues regardless of any of this. Plotinus teaches at Rome itself through the worst of the crisis, developing in lecture the mystical, intensely systematic metaphysics his student Porphyry later edits into the Enneads, a philosophy almost entirely uninterested in the political chaos surrounding its composition. Diophantus, at Alexandria, develops an abbreviated or syncopated algebraic notation for unknowns and powers in the Arithmetica; Liu Hui, in a fractured China, refines mathematical approximations of pi and writes on surveying technique. Neither the Roman soldier-emperors cycling through power nor the warlords carving up Han China's territory much interrupt the actual working mathematics and philosophy of the age, a continuity between intellectual and political history that the crisis, for all its violence, does not manage to break. Farther still, Teotihuacan in the Valley of Mexico continues its own rise into one of the largest cities in the Americas of its time, its grid of walled apartment compounds rising without any awareness that Rome or China exist.