Li Jing
Sometime after 626 (after Li Shimin became emperor, succeeding his father Emperor Gaozu), Su became a military officer under the prominent general Li Jing. In Li Jing's campaign against Eastern Turkic Khaganate in 629–630, in the final decisive battle where Li Jing crushed the forces of the Jiali Khan Ashina Duobi and killed his wife (Sui's Princess Yicheng) in battle, Su served as Li Jing's forward commander. After Li Jing returned to the Tang capital Chang'an victoriously, Su was promoted to a mid-level officer position.