By the autumn of 618, the Sui dynasty existed in name only. Emperor Yang had abandoned the north two years earlier, withdrawing to Jiangdu in the south, where he was assassinated by his own general Yuwen Huaji. Nine major contenders carved up what remained. Li Yuan had seized the Sui capital of Chang'an and proclaimed the Tang dynasty. Dou Jiande controlled much of Hebei. And Li Mi, commanding a volatile coalition of rebel peasants and Sui defectors, held most of Henan -- everything, that is, except Luoyang itself. Wang Shichong, nominally a Sui loyalist, clung to the eastern capital with the remnants of the imperial administration and a grandson of the dead emperor installed as a puppet ruler. The confrontation between Wang and Li Mi would determine who controlled the most populous region of the former empire.

Li Mi pursued Wang to the hills north of Yanshi and encamped with roughly 40,000 men -- double Wang's force. His position was strong: the Mang hills gave him high ground, and Wang's army was cut off between the Luo River, irrigation canals, and Li Mi's own troops. Sun Tzu had written about exactly this kind of desperate enemy -- cornered men fight with the courage of those who have nothing to lose -- and Li Mi knew the text well. He urged patience: let Wang's supplies run out, then sweep in. But his generals disagreed. They saw only an exhausted, outnumbered enemy and demanded immediate battle. Li Mi understood that his coalition army was held together by the loyalty each warlord commanded from his own followers, not by any allegiance to him. He could not afford to be seen as timid. Against his better judgment, he agreed to fight.

Wang's tactical genius showed itself that evening. He sent cavalry to attack a camp on the plain -- a diversion that drew Li Mi's attention east. Under cover of darkness, 200 horsemen rode around behind the hills and concealed themselves in a ravine north of Li Mi's main camp. Meanwhile, Wang's engineers laid bridges across the irrigation canal. Before dawn, his entire army crossed and deployed in battle formation within striking distance of the rebel camps. At first light, Li Mi's troops woke to find an enemy army already formed and advancing. The camps were unfortified -- Li Mi had trusted the terrain to protect him and neglected basic field fortifications. As his men scrambled to form a battle line, Wang signaled. The hidden cavalry charged down from the north, setting fire to the camp. Smoke rising behind them, the army in front of them -- Li Mi's coalition shattered and ran.

Li Mi escaped with roughly 10,000 men, but his authority was destroyed. His followers flocked to Wang Shichong, who quickly became master of territories stretching from the sea to the Yangzi River. Li Mi fled to the Tang court in Chang'an, where he was later executed. Wang, meanwhile, deposed the puppet Sui emperor Yang Tong in May 619 and declared himself emperor of a new Zheng dynasty. His reign proved brief and brutal. Repressive rule cost him popular support, and in 621 the Tang prince Li Shimin laid siege to Luoyang. Wang's ally Dou Jiande marched to relieve him, but Li Shimin destroyed Dou's army at the Battle of Hulao on May 28, 621. Wang surrendered four days later and was killed on the road to exile. The Tang dynasty's unification of China had begun in earnest.