Hangu Pass
During the early Warring States period, as its neighbours in the Central Plains began rapidly developing, Qin was still in a state of underdevelopment and decline. The Wei state, formed from the Partition of Jin, became the most powerful state on Qin's eastern border. Qin most relied on natural defenses, such as the Hangu Pass (函谷關; northeast of present-day Lingbao, Henan) and Wu Pass (武關, in present day Danfeng County) in the east, to protect its Guanzhong heartland. Between 413 and 409 BC during the reign of Duke Jian of Qin, the Wei army led by Wu Qi, with support from Zhao and Han, attacked Qin and conquered some Qin territories west of the Yellow River.