into a trap by Qi forces. In 307 BC, King Wuling of Zhao ordered the adoption of nomadic clothing in order to train his own division of cavalry archers. In the field of military planning, the niceties of chivalrous warfare were abandoned in favor of a general who would ideally be a master of maneuver, illusion, and deception. He had to be ruthless in searching for the advantage, and an organizer in integrating units under him. Qin–Han In 221 BC, the Qin unified China and ushered in the Imperial Era of Chinese history. Although it only lasted 15 years, Qin established institutions that would last for millennia. Qin Shi Huan, titling himself as the "First Emperor", standardized writing systems, weights, coinage, and even the axle lengths of carts. To reduce the chance of rebellion, he made the private possession of weapons illegal. In order to increase the rapid deployment of troops, thousands of miles of roads were built, along with canals that allowed boats to travel long distances. For the rest of Chinese history, a centralized empire was the norm. During the Qin dynasty and its successor, the Han, the Chinese armies were faced with a new military threat, that of nomadic confederations such as the Xiongnu in the North. These nomads were fast horse archers who had a significant mobility advantage over the settled nations to the South. In order to counter this threat, the Chinese built the Great Wall as a barrier to these nomadic incursions, and also used diplomacy and bribes to preserve peace. Although the Qin general Meng Tian ousted the Xiong-nu from the Ordos region, they regained power under the rule of Maodun. Maodun conquered the Eastern Hu and drove the Yuezhi tribes west. He reclaimed the Ordos from the now crumbling Qin empire and defeated the first Han emperor Gao in battle. This led to a policy of appeasement until the reign of Wudi of Han, who decided to take a tougher stance. However, protecting the borders required a significant investment. Manning the stations of the Great Wall took about ten thousand men. To support them, fifty to sixty thousand soldier-farmers were moved to the frontiers in order to reduce the cost of transporting supplies. These drafted farmers were not good cavalry troops, so a professional army emerged on the frontiers. These consisted of northern Han mercenaries, convicts working for their freedom, and subjected