In May 290, when Sima Xiao was about 20 years old, Emperor Wu died. Emperor Wu's successor, Emperor Hui was
developmentally disabled; his reign saw a series of regents who ruled on his behalf. Xiao's activities during the regencies of
Yang Jun (father of Emperor Wu's second wife
Empress Yang Zhi),
Sima Liang and
Wei Guan (who were co-regents), Empress
Jia Nanfeng (Emperor Hui's first wife) and
Sima Lun (Prince of Zhao) were poorly documented.
War of the Eight Princes After
Sima Jiong was executed after a battle against
Sima Ai in January 303,
Liu Kun and his family went to serve Sima Xiao. While
Emperor Hui of Jin was held hostage in
Chang'an, Sima Xiao approached
Zu Ti and invited Zu to serve him, but he declined. In August 305,
Sima Yue sent out a proclamation throughout the regions east of Luoyang calling for a campaign against
Sima Yong. He cited that
Zhang Fang had forcibly moved Emperor Hui to
Chang'an and aimed to bring him back to Luoyang. His brothers and several other prominent governors such as
Wang Jun and Sima Xiao all joined him, and he also began handing out new appointments to his allies without the emperor's assent. Among the forces in the east, only the Inspector of
Yu province,
Liu Qiao and the self-declared Inspector of
Yan province,
Sima Mao sided with Sima Yong after Yue attempted to remove them from their positions in September, with Liu Qiao accusing the prince of overstepping his authority. Liu Qiao sent his troops to block Yue from advancing west at
Xiao County. On 20 November, Yong appointed Zhang Fang commander of 100,000 troops and sent him to assist Liu Qiao. Soon, Liu Qiao captured
Xuchang from Sima Xiao, prompting him to flee to
Ji province. Due to the early setbacks, a general of Yue,
Chen Min, received permission to go east to recruit more soldiers. However, once there, Chen Min instead rebelled and took control of the
Jiangnan region. In Ji province, Sima Xiao received some elite
Xianbei and
Wuhuan cavalry forces from Wang Jun. With them, Xiao and his general,
Liu Kun launched a successful counterattack on Yong and Liu Qiao's forces in January 306, killing
Shi Chao at Xingyang. Xiao's forces then routed Sima Mao at Linqiu (廩丘, in present-day
Puyang,
Henan) and forced him to flee back to his fief in
Dongping. Then, they won a great victory over Liu Qiao at
Qiao Commandery, causing his army to collapse. After Sima Yue defeated
Sima Yong later in 306 and welcomed Emperor Hui back to Luoyang on 28 June, for his merits in the war, in September 306, Sima Xiao was appointed Minister of Works and transferred to Yecheng. After his victory, Sima Yue put out an order for
Sima Ying's arrest, and Sima Ying fled with his sons Sima Pu (司馬普) the Prince of Lujiang and Sima Kuo (司馬廓) the Prince of Zhongdu, attempting to flee to Gongshi Fan (公師藩), when he was intercepted by Feng Song (馮嵩) the governor of Dunqiu Commandery (頓丘, roughly modern Puyang, Henan) and taken to Yecheng. Sima Xiao imprisoned Sima Ying but did not wish to kill him. ==Death==