After Cao Cao occupied Hanzhong after suppressing Zhang Lu in 215,he immediately divided the Han River Valley into four prefectures (known as 'Jun') according to topographical features.Compared to the Hanzhong Prefecture which was located in the Hanzhong Basin,the other three ones,which were east of Hanzhong and were located in Xikang Basin and Exi mountainous area,were referred to as 'East Three Jun'.During the fight for Hanzhong between Wei and Shu States,East Three Jun was once occupied by Shu State for a short period and soon returned to Wei State again due to a general of Shu State—Meng Da's betrayal.Bordering on two hostile states in the west and south,East Three Jun held an important strategic position though its geographical and traffic conditions were a hinderance to military operations.Wei State,therefore,frequently regulated the administrative regionalization in this frontier area and sophisticatedly adapted to the changing situations.In this manner Wei State was able to achieve its strategic targets in this region,both in military affairs and to some degree in local development.