Defense commissioners (fangyushi 防禦使, also written 防御使), full title defense and security commissioner (fangyu shouzhuo shi 防禦守捉使), were high military officials dispatched by the central government and entrusted with the temporary supervision of military affairs. According to the size of the area under their jurisdiction, there were chief defense commanders (du fangyushi 都防禦使) and prefectural defense commanders (zhou fangyushi 州防禦使). The last title was first seen in 698, when the commander-in-chief (dudu 都督) of the prefecture of Xiazhou 夏州 was appointed defense commander of Yanzhou 鹽州. In 714 Xue Ne 薛訥 (649-720) was appointed defense commissioner of Longyou 隴右.
During the rebellion of An Lushan 安祿山, Emperor Xuanzong 唐玄宗 (r. 712-755) of the Tang dynasty 唐 (618-907) decided to appoint defense commissioners in militarily important regions. In 756 Xu Yuan 許遠 (709-757) was made governor (taishou 太守) of the commandery of Suiyang 睢陽 and concurrently defense commissioner. He was just the first of several defense commissioners appointed in the circuits of Henan 河南, Hebei 河北, Hedong 河東, Guannei 關內, Shannan 山南 and Jiannan 劍南. Their mandates were nullified in 762, yet Emperor Daizong 唐代宗 (r. 762-779) soon appointed new ones. The office remained in use until the mid-10th century.
Chief defense commanders supervised several prefectures (zhou 州), yet their status was lower than that of military commissioners (jiedushi 節度使). Their status raised with the growing power of military commissioners in the late Tang period. The office of defense commissioner was often taken over by regional inspectors (cishi 刺史) or surveillance commissioners (guanchashi 觀察使). They were in fact the highest military officials of one prefecture. In some prefectures the office was called military training commissioner (tuanlianshi 團練使).
In the Liao 遼 (907-1125) and Jin 金 (1115-1234) empires the title was also used, but denoted a temporary office with military and civilian duties. It was closer to a kind of high police officer than to a military officer. The Yuan dynasty 元 (1279-1368) also made use of it and in 1357 ordered that all prefects may concurrently hold the title of defense commissioner. In that function the local administrators were responsible for the suppression of rebellions.
The word fangyu 防御 was later used by the Qing dynasty 清 (1644-1911) to denote a lower rank of officer of the Green Standard troops (lüying 綠營), like platoon commander. It had nothing to do with the defense commissioner of older times.