阴山
In 625, khagan attacked Ling Prefecture (靈州, roughly modern Yinchuan, Ningxia). Emperor Gaozu sent the general Zhang Jin (張瑾) to resist Tujue with Wen Yanbo serving as Zhang's secretary general. When Zhang engaged khagan, however, he suffered a major defeat, and Wen was captured. As Wen was an important official, khagan put him under interrogation and asked him about the strengths and weaknesses of the Tang state. Wen refused to answer and khagan moved him to the Yin Mountains region.