Hangu Pass (Traditional Chinese: 函谷關; pinyin: Hángǔ Guān) was a strategic pass in ancient China just south of the great eastern bend of the Yellow River in today's Lingbao, Henan Province. The State of Qin built the pass in 361 BC as its eastern gate.
Because of its strategic location between the ancient lands of the State of Qin and the central plains of China, many famous ancient battles were fought at Hangu Pass. During the Warring States Period, Hangu Pass was heavily defended by the State of Qin, as the pass was the only reasonable route to invade the State of Qin from the central plains of China. Hangu Pass was also a vital route in invading the ancient capital of Luoyang from the west.
According to legend, Lao Zi supposedly wrote the Dao De Jing at Hangu Pass.