刘武周
In summer 619, Liu Wuzhou launched an attack on the Taiyuan region, and Li Yuanji sent the general Zhang Da (張達) to resist Liu, despite Zhang's protestations that the army given him was too small to resist Liu. When Zhang was in fact defeated, he surrendered to Liu and served as Liu's guide in subsequent attacks. Liu soon put Taiyuan under siege, and while Li Yuanji initially repelled him, Liu was soon back and again had Taiyuan under siege. In fall 619, Li Yuanji fled with his wife and concubines back to Chang'an, and Liu captured Taiyuan, moving his capital from Mayi (馬邑, in modern Shuozhou, Shanxi) there. (In 620, Li Shimin defeated Liu and recaptured the region.)