[Why] In late High Tang, the equal-field system collapsed, the militia system disintegrated, and military governors' power proliferated. In his late reign, Emperor Xuanzong became arrogantly lax in governance, allowing the three-governorship commander An Lushan to gather heavy troops with intent to seize the central regime. [What] An Lushan and Shi Siming raised troops against Tang at Fanyang, with their iron cavalry driving deep to seize Luoyang and Chang'an. Xuanzong was forced to flee to Bashu, the empire was largely engulfed in fire, and the prosperous Central Plains were reduced to ruins. [Who] An Lushan was the rebellion's ambitious leader who used Hu cavalry mobility to grievously wound the empire. Veteran generals like Guo Ziyi and Li Guangbi by their loyalty and supreme command at last with great difficulty recovered the two capitals. [How] This turmoil definitively shattered the apex of High Tang all-nations-come-to-court prosperity and forced the empire's political center to contract inward. It became the painful turning point of Chinese medieval history from open cosmopolitan to inward-looking.
Why
The historic event of An Lushan Rebellion represents a key developmental peak of the Huaxia dynastic system. The tragic structural decline of the cosmopolitan empire. By establishing this moral or administrative benchmark, it continues to shape the structural and philosophical fabric of ancient Chinese statecraft.