Battle of Huoyi: Li Yuan defeats a Sui dynasty army, opening the path to his capture of the imperial capital Chang'an and the eventual establishment of the Tang dynasty.

The Battle of Huoyi was fought in China on 8 September 617, between the forces of the rebel Duke of Tang, Li Yuan, and the army of the ruling Sui dynasty. Li Yuan, with an army of around 25,000, was advancing south along the Fen River towards the imperial capital, Daxingcheng. His advance was stalled for two weeks due to heavy rainfall, and he was met at the town of Huoyi by an elite Sui army of 20,000 men. Li Yuan's cavalry, under the command of his two eldest sons, lured the Sui out of the protection of the city walls, but in the first clash between the two main armies, Li Yuan's forces were initially driven back. At that point, possibly due to a stratagem on Li Yuan's behalf, the arrival of the rest of the rebel army, or the flanking maneuver of Li Yuan's cavalry, which had gotten behind the Sui army, the Sui troops collapsed and routed, fleeing back towards Huoyi. Li Yuan's cavalry, however, cut off their retreat. The battle was followed by the capture of weakly defended Huoyi and the advance on Daxingcheng, which fell to the rebels in November. The following year, Li Yuan deposed the Sui and proclaimed himself emperor, beginning the Tang dynasty.

Emperor Gaozu of Tang, born Li Yuan, courtesy name Shude, was the founding emperor of the Tang dynasty of China, reigning from 618 to 626 CE. Under the Sui dynasty, Li Yuan was the governor in the area of modern-day Shanxi, and was based in Taiyuan.

The Sui dynasty was a short-lived Chinese imperial dynasty that ruled from 581 to 618. The re-unification of China proper under the Sui brought the Northern and Southern dynasties era to a close, ending a prolonged period of political division since the War of the Eight Princes. The Sui endeavoured to rebuild the country, re-establishing and reforming many imperial institutions; in so doing, the Sui laid much of the foundation for the subsequent Tang dynasty, who after toppling the Sui would ultimately preside over a new golden age in Chinese history. Often compared to the Qin dynasty (221–206 BC), the Sui likewise unified China after a prolonged period of division, undertook wide-ranging reforms and construction projects to consolidate state power, and collapsed after a brief period.