Juqu Mujian
In 439, Emperor Taiwu, even though by this point he had taken Juqu Mengxun's daughter as a concubine and had married his sister Princess Wuwei to Juqu Mengxun's son and successor Juqu Mujian, became resolved to conquer Northern Liang, and Cui greatly encouraged him, despite opposition from other key officials, including Li Shun, Daxin Jin (達奚斤), and Tuxi Bi (吐奚弼). (Why Li switched his position from supporting a campaign to opposing it at this point was unclear, but Cui would later accuse him of having accepted bribes from Juqu Mengxun and Juqu Mujian.) Li and Tuxi argued that Northern Liang's territory was desolate, and that the Northern Wei army would run out of food and water. Emperor Taiwu followed Cui's suggestion, and was able to quickly conquer Northern Liang and force Juqu Mujian's surrender—and when he saw that the region around Northern Liang's capital Guzang (姑臧, in modern Wuwei, Gansu) was exceptionally fertile, he became very resentful of Li, and would eventually force Li to commit suicide in 442. Meanwhile, after the conquest of Northern Liang, at Cui's request, Juqu Mujian's officials Yin Zhongda (陰仲達) and Duan Chenggen (段承根) were added to Cui's staff of historians. He also added Gao Yun to his staff around this time.