窦建德
Following victories in the west that had established his credentials as a general, in August 620 Li Shimin marched against Wang Shichong. Tang troops blockaded Wang in his capital of Luoyang, while seizing the rest of Henan province. After failing in his efforts to break through the Tang siege, and suffering from ever greater privations, Wang solicited help from Dou Jiande. In April 621, Dou Jiande led a 100,000–120,000 strong army west to confront the Tang. Li Shimin's generals urged him to retreat west and protect the Tang core territory at Shanxi, but doing so would surrender the northeastern plains, at that time the heartland of China, to Dou. Consequently, Li Shimin took a gamble by leading a small force east to occupy the strategic Hulao Pass, while the bulk of his army was left behind continuing the siege of Luoyang. Ensconced in favourable defensive positions, the Tang managed to hold up the Xia advance. A large and heterogeneous army, the Xia lacked the flexibility to either outflank Li Shimin's position or abandon the Luoyang campaign and attack the exposed Tang heartland at Shanxi. As a result, the standoff between the two armies continued for several weeks.