Dou Jiande
Li Shimin was Tang's most capable general in its campaigns to reunite China following Sui's collapse, defeating the major enemies Xue Rengao the Emperor of Qin, Liu Wuzhou the Dingyang Khan, Wang Shichong the Emperor of Zheng, and Dou Jiande the Prince of Xia. In doing so, he overshadowed his older brother Li Jiancheng, the crown prince. The brothers developed an intense rivalry. Princess Zhangsun was said to serve her father-in-law Emperor Gaozu carefully while trying to reduce the adversarial nature of the brothers' relationship (unsuccessfully).