In describing the Western Regions, History of the Han Dynasty mentions the establishment of four prefectures and two passes. It was one of the great achievements made by the dynasty after taking over the Hexi Corridor from the hands of the Xiongnu. The two passes were located in the west of Dunhuang. The four prefectures include Wuwei and Jiuquan set up in second year of Yuanshou (121 BC) when the Hexi Corridor was captured, under the jurisdiction of the latter Dunhuang was placed. Later in the sixth year of Yuanding (111 BC) the prefectures were reorganized into four, adding Zhangye and Dunhuang. Dunhuang was originally a very small division separated from Jiuquan for the purpose of westward military actions and the expansion of irrigated agricultural areas. However, since it occupied a considerable important position geographically, it developed very fast.