Fu Jian lost an empire because he forgot his dying prime minister's two rules

By 376 CE the Former Qin emperor Fu Jian had unified northern China under a single regime for the first time in decades. His prime minister, the strategist Wang Meng, had built the bureaucracy, reformed the military, and created the state that made unification possible. In 375 CE, on his deathbed at age 51, Wang Meng gave Fu Jian two warnings: never attack the Eastern Jin, which held the Yangtze valley and represented the legitimate Han dynasty; and eliminate the Xianbei and Qiang nobles within Former Qin, who were conquered enemies waiting for their moment.

Fu Jian rejected both. He saw his army of 870,000 as invincible. He said the Eastern Jin 'could not resist our troops any more than autumn leaves resist the wind.' His brother Fu Rong and most of his court argued against the campaign, citing Wang Meng's warning. Fu Jian overruled them all. In 383 CE he launched a full-scale invasion of Eastern Jin.

At the Fei River, the Jin commander Xie Xuan offered Fu Jian's army room to cross so the two sides could fight a pitched battle. Fu Jian ordered a general retreat to open space — and the retreat became a rout when a captured Jin officer shouted that the Qin army was defeated. The 870,000 dissolved in panic. Fu Rong was killed trying to rally the troops. Jin forces suffered perhaps 5,000 dead; Former Qin lost most of its army to combat, starvation and exposure.

Wang Meng's second warning arrived on schedule. The Xianbei and Qiang leaders under Fu Jian — Murong Chui, Yao Chang — declared independence within months of the defeat. Chang'an fell in 385 CE. Fu Jian was captured and killed by Yao Chang, his former general. Former Qin fragmented into a dozen warring states. China remained divided for another two centuries.