Hangu Pass
The river left these paths in 602 BC and shifted several hundred kilometers to the east. Sabotage of dikes, canals, and reservoirs and deliberate flooding of rival states became a standard military tactic during the Warring States period. As the Yellow River valley was the major entryway to the Guanzhong area and the state of Qin from the North China Plain, Qin heavily fortified the Hangu Pass; it saw numerous battles and was also an important chokepoint protecting the Han capitals of Chang'an and Luoyang. Major flooding in AD 11 is credited with the downfall of the short-lived Xin dynasty, and another flood in AD 70 returned the river north of Shandong on essentially its present course.