Emperor Taizong of Tang (r. 626-649), the second emperor of the Chinese Tang Dynasty, had subjugated the Xianbei state Tuyuhun in 635.
Thereafter, Tuyuhun's southwestern neighbor, the Tibetan state Tufan, had risen in power and soon displaced Tuyuhun as the major threat to Tang's west.
In 638, in retaliation to Emperor Taizong's refusal to give him a Tang princess in marriage, Tufan's Songtsän Gampo, believing that the refusal is due to Tuyuhun interference, attacks Tuyuhun and Tang with a grand force of 200,000, and while Tang's 50,000 forces, commanded by Niu Jinda, serving under Hou Junji, repel the Tufan attack, Emperor Taizong decides to placate Songtsän Gampo by giving him a distant niece, Princess Wencheng, in marriage.
For the rest of Emperor Taizong's reign, there will be no further major battles between Tang and Tufan, although Tufan will pose major military threats for almost the rest of Tang Dynasty rule.
