In 25 AD, Emperor Guangwu of the Han Dynasty, Liu Xiu, after pacifying the Central Plains and establishing the Eastern Han, turned his attention to the regional warlords in the remote borderlands, driven by his grand ambition to unify all of China.
At that time, the two most threatening and powerful local separatist forces to the Eastern Han Dynasty were Gongsun Shu, who ruled Bashu, and Wei Xiao, who dominated Longxi.
In 32 AD, Emperor Guangwu personally led a campaign against the warlord Wei Xiao in Longxi. Realizing he could not defeat the Han army, Wei Xiao fortified the besieged city of Xi and sent a messenger to Gongsun Shu for reinforcements.
Gongsun Shu, knowing that Emperor Guangwu would attack Shu once he conquered Longxi, sent reinforcements to garrison Shanggui. When the emperor learned of this, he dispatched troops to assault Shanggui, but its defenses were so formidable that a swift victory proved impossible.
Emperor Guangwu, seeing the two cities still unconquered after a long siege, returned to Luoyang. Before leaving, he issued an edict to his general Cen Peng: "Once both cities fall, lead the army south to attack Shu. People suffer from not knowing contentment—having pacified Long, they now set their sights on Shu."
These words mean: "If the general captures two cities, he can move south to attack Shu. People are never satisfied—having pacified Long, they then hope to pacify Shu!"
Cen Peng understood this was Emperor Guangwu's expectation of him, so he devised a plan to flood the western city, intending to capture it before attacking Shanggui. However, the flooding failed, and Shanggui's defenders sent reinforcements to the western city. Seeing the enemy outnumbered his forces, Cen Peng had no choice but to retreat.
Later, Wei Xiao's son Wei Chun surrendered to Emperor Guangwu, and Longxi was finally pacified. Cen Peng was then ordered to lead troops to attack Shu, quickly conquering it, fulfilling Emperor Guangwu's wish to gain Longxi and then Shu.
Later, the idiom "Gain Long, Eye Shu" came to describe wanting another after obtaining one, metaphorically meaning insatiable greed.
Source: *Book of the Later Han*, "Biography of Cen Peng"
Meaning of the Idiom: Later, the Chinese idiom "得陇望蜀" came to describe wanting another after obtaining one, metaphorically meaning insatiable greed.