In 278 BCE, King Zhao of Qin planned to attack the state of Wei, but a strategist advised against it, arguing, "Striking Wei is like cutting the spine of the six eastern states—if you hit a snake's tail, its head will defend it; if you hit its head, its tail will strike back. Attacking Wei would force the six states to unite like a snake coiling to protect its middle, and their combined strength could turn the tide against Qin, bringing disaster to your doorstep."
The strategist argued that attacking Wei was unwise, suggesting instead to march south against Chu. Among the six states, Chu's military was weaker, and attacking it would prevent other states from coming to its aid. Historically, striking the weaker state first was a proven strategy. The strategist advised:
Before attacking Jie, the last ruler of the Xia Dynasty, Tang of Shang first subdued the small state of Mixu, using it to train and sharpen his forces. Once he seized Mixu, Tang knew he could tame Xia Jie. Qin's strategy to strike Chu first follows the same logic. If Qin wishes to conquer the feudal lords, attacking the powerful central state of Wei without first practicing on a weaker foe would only drain its own strength. By targeting Chu first, other states cannot come to its aid, Qin will win, expand its territory, enrich the nation, strengthen its army, and earn the respect of all under heaven.
King Zhao of Qin, following his strategist's advice, marched south from Lantian to attack the Chu capital, Ying.
Later, the idiom "head and tail rescue each other" came to be used as a metaphor for mutual aid.
Source: *Strategies of the Warring States*, Chapter "Strategies of Wei IV"
Meaning of the Idiom: Later, the Chinese idiom "首尾相救" came to describe how mutual aid works.