[Why] After Mongols entered the Central Plains, they faced the severe governance challenge of a population of only a million ruling nearly a hundred million Han. Yuan rulers continued the Mongol Empire's conquest-rank tradition and applied differentiated political control to the vast conquered ethnic groups. [What] The Yuan court divided the population into four ranks: Mongols, Semu (allied Western Regions peoples), Han people (northern), and Southerners. Each rank was treated very differently in examinations, official appointment, criminal punishment, and taxation, with Han and Southerners strictly limited to junior offices. [Who] Kublai was the highest decision-maker of the system design, balancing the demands of steppe aristocrats and Han-land bureaucrats. Han hereditary marquises were partly incorporated into the Yuan bureaucracy but never reached the core of imperial power. [How] The Four-Class System maintained the Mongol minority's ruling stability in the short term but also sharpened deep ethnic conflict. It ultimately became the core social mobilization symbol of the Red Turban uprisings and accelerated the collapse of the Great Yuan.
Why
The historic event of Four-Class Social System represents a key developmental peak of the Huaxia dynastic system. The differentiated political control of vast conquered peoples by a Mongol ruling minority. By establishing this moral or administrative benchmark, it continues to shape the structural and philosophical fabric of ancient Chinese statecraft.