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 . Initial contacts between Tang and Tufan . The existence of Tufan was unknown to the Chinese until 634 , when Tufan ' s ruler Songtsän Gampo sent emissaries with tributes to China ' s Tang Dynasty and requested that he be allowed to marry a Tang princess . Emperor Taizong of Tang , who was then battling Tufan ' s neighbor Tuyuhun , situated between Tang and Tufan , did not initially respond to Songtsän Gampo ' s marriage overture but did sent the emissary Feng Dexia ( 馮德遐 ) to Tufan to establish peaceful relations . Conflict in 638 . Meanwhile , in late 634 , Emperor Taizong had sent the general Li Jing against Tuyuhun and , in a major campaign , overpowered Tuyuhun ' s Busabo Khan Murong Fuyun , who was killed in flight . Tang thereafter created Murong Fuyun ' s son Murong Shun as Tuyuhun ' s khan and , after Murong Shun was assassinated late in 635 , supported Murong Shun ' s son Murong Nuohebo as khan . Feng Dexia appeared to have gotten to Tufan around the same time . By this time , Songtsän Gampo had known that , in the past , Eastern Tujue and Tuyuhun khans had married Chinese princesses , and therefore sent an emissary to accompany Feng back to Tang with further tributes and requesting to marry a Tang princess . Emperor Taizong refused . When the Tufan emissary returned to Tufan , he informed Songtsän Gampo that Emperor Taizong had initially agreed to the marriage proposal , but that Murong Nuohebo had visited Tang and was interfering , leading to Emperor Taizong ' s refusal . Songtsän Gampo , believing in the report , attacked Tuyuhun in anger , capturing much of the Tuyuhun people and forcing the rest to flee north of the Qinghai Lake . Meanwhile , in fall 638 , Tufan forces , numbering some 200 , 000 and apparently commanded by Songtsän Gampo himself , began attacking Tang itself , first attacking Song Prefecture ( 松州 , roughly modern Ngawa Tibetan and Qiang Autonomous Prefecture , Sichuan ) - - but meanwhile sending emissaries to the Tang capital Chang ' an , again offering tributes and declaring that they were intending to welcome a princess . Emperor Taizong responded by commissioning the general Hou Junji to command an army , assisted by the generals Zhishi Sili ( 執失思力 ) , Niu Jinda , and Liu Jian ( 劉簡 ) . Meanwhile , Tufan forces were putting Song Prefecture ' s capital under siege , but Tang advance forces , commanded by Niu , surprised them and defeated them . In fear , Songtsän Gampo withdrew and sent emissaries to Chang ' an to apologize — but to again request marriage . Emperor Taizong agreed this time . Marriage between Songtsän Gampo and Princess Wencheng . However , no further action was taken to carry out the marriage for about two years . In fall 640 , Songtsän Gampo sent his prime minister Lu Dongzan ( 祿東贊 ) to Tang to offer tributes of gold and jewels , again requesting marriage . In response , Emperor Taizong created a daughter of a clansman the Princess Wencheng , preparing to give her to Songtsän Gampo in marriage . Impressed by Lu Dongzan ' s propriety in interacting with him , he also , over Lu Dongzan ' s own objection — that he already had a wife and that it would be inappropriate for him to marry before his king — gave Lady Duan , the granddaughter of Princess Langye , to Lu Dongzan as a wife as well . In spring 641 , Emperor Taizong sent his cousin , Li Daozong the Prince of Jiangxia , to accompany Lu Dongzan back to Tufan and to escort Princess Wencheng . When they arrived in Tufan , Songtsän Gampo was said to be so pleased that he bowed to Li Daozong , using ceremony appropriate for a son - in - law toward a father - in - law . He built a palace for Princess Wencheng and changed into Chinese clothing before he met her . It was said that at that time , the Tufan people had a custom that Princess Wencheng hated — that people would paint their faces red — and that he prohibited the custom for her sake . Other points . Thanks to another side of the agreement , he also sent nobles and family members to Chang ' an to study at Tang ' s imperial university . In an old custom which make this people are de facto use as hostages by Tang , while they also learn Chinese custom and culture for better future relationships . Songtsän Gampo also requested Chinese scholars . Early in Gaozong ' s reign , Tibet also requested technology transfers for sericulture , wine - brewing , grain mill s and paper industries . For the rest of Emperor Taizong ' s reign , there would be no further records of conflicts with Tufan . In 647 , for example , when Tang forces attacked the Xiyu state Qiuzi ( 龜茲 ) ' s king Bai Helibushibi ( 白訶黎布失畢 ) , Emperor Taizong requisitioned Tufan troops . Moreover , in 648 , when the Tang emissary Wang Xuance became stuck in political turmoil of an Indian state , he sought aid from both Tufan and Nepal and received aid from both in helping to defeat one of the factions ( 649 ) . There would not be a conflict between Tang and Tufan until 662 , a dozen years after both Emperor Taizong and Songtsän Gampo had died .