Query: What surrounds the Sichuan basin to the west?
Context: The rulers of the expansionist Qin dynasty, based in present-day Gansu and Shaanxi, were only the first strategists to realize that the area's military importance matched its commercial and agricultural significance. The Sichuan basin is surrounded by the Himalayas to the west, the Qin Mountains to the north, and mountainous areas of Yunnan to the south. Since the Yangtze flows through the basin and then through the perilous Yangzi Gorges to eastern and southern China, Sichuan was a staging area for amphibious military forces and a refuge for political refugees.[citation needed]