How did Li Shimin help the Tang dynasty gain control of China’s eastern plain?

Li Shimin, later known as the Taizong emperor, played a decisive role in securing the eastern plain for the Tang dynasty through his military leadership and strategic planning.12

By 621, Li Shimin was leading the siege of Luoyang, the stronghold of the rival leader Wang Shichong. When Wang attempted to form a strategic alliance with Dou Jiande—a powerful rebel leader controlling much of Hebei—Li Shimin preemptively intervened. In the spring of 621, he attacked Dou’s approaching army before it could break the siege, successfully routing the forces and capturing Dou.13

Following this victory, Wang Shichong capitulated. By eliminating these two primary rivals, Li Shimin enabled the Tang to extend their control over the eastern plain, which was the most populous and prosperous region of China at the time. Subsequently, in 621, his father, the Gaozu emperor, granted him authority over both military and civil administration in the east, allowing him to consolidate power from his headquarters in Luoyang.132