The state of Qin fortified the pass in 361BC as its eastern border, protecting access to their homeland from the armies of the other Warring States competing to succeed the Zhou. It continued to protect the Guanzhong area of the Qin and Western Han empires. Under the Eastern Han that succeeded Wang Mang's "Xin dynasty", its fortifications protected the capital Luoyang in the other direction, staving off attacks coming from the west and northwest.
After the fall of the Han and rise of the Three Kingdoms, the Hangu Pass lost most of its importance as the primary fortifications moved a little west to Tongguan.
In Kingdom manga and anime, this pass was the last chokepoint Li Mu and his 5 allied Coalition Army in order to defeat Qin. Their attacks was repelled with great losses, forcing Li Mu to make a decision of attacking Xianyang via Zui.