In 121 BC, Han forces expelled the Xiongnu from a vast territory spanning the Hexi Corridor to Lop Nur. They repelled a joint Xiongnu-Qiang invasion of this northwestern territory in 111 BC. In that year, the Han court established four new frontier commanderies in this region: Jiuquan, Zhangyi, Dunhuang, and Wuwei. The majority of people on the frontier were soldiers. On occasion, the court forcibly moved peasant farmers to new frontier settlements, along with government-owned slaves and convicts who performed hard labor. The court also encouraged commoners, such as farmers, merchants, landowners, and hired laborers, to voluntarily migrate to the frontier.
Try to answer this question if possible (otherwise reply "unanswerable"): In what year did Han forces stop a joint Xiongnu-Qiang invasion?
111 BC