The Hanzhong basin sits between the Qinling and the Daba mountains. South lies Chengdu. North lies Guanzhong. Zhang Lu held a Way of the Five Pecks of Rice regime here until Cao Cao took it in 215. Liu Bei then fought Cao Cao for the basin. In 219 Xiahou Yuan died at Dingjun Mountain, Cao Cao withdrew, and Liu Bei, holding Hanzhong, became King of Hanzhong.
The campaign is the battle of Hanzhong. The royal title was not only new land. It was the name Liu Bei used against Wei before he was emperor in Chengdu.
When Zhuge Liang opened the northern expeditions, troops and grain moved from Hanzhong through Qishan or the Xie valley. Jieting spoiled one attempt to step from Hanzhong toward Guanzhong. It was not an attack on Chengdu. The novel often passes over the basin. Geographically it is the whole reason Shu could look toward Chang’an.