Emperor Taizong's Campaign Against Tufan
Emperor Taizong of Tang (r. 626-649), the second emperor of Chinese dynasty Tang Dynasty, subjugated the Xianbei state Tuyuhun in 635. Thereafter, Tuyuhun's southwestern neighbor, the Tibetan state Tufan, rose 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 was due to Tuyuhun interference, attacked Tuyuhun and Tang with a grand force of 200,000, and while Tang's 50,000 forces, commanded by Niu Jinda (牛進達), serving under Hou Junji, repelled the Tufan attack, Emperor Taizong decided to placate Songtsän Gampo by giving him a distant niece, Princess Wencheng, in marriage. For the rest of Emperor Taizong's reign, there would be no further major battles between Tang and Tufan, although Tufan would pose major military threats for almost the rest of Tang Dynasty.
Read more about Emperor Taizong's Campaign Against Tufan: Initial Contacts Between Tang and Tufan, Conflict in 638, Marriage Between Songtsän Gampo and Princess Wencheng, Other Points