During the conflict between Han and Chu, Han King Liu Bang seized the moment when Xiang Yu was leading his main army to attack Qi King Tian Rong. Following his strategist Chen Ping's plan, Liu Bang marched eastward, striking directly at Xiang Yu's base in Pengcheng (modern-day Xuzhou, Jiangsu) and quickly captured it.
When Xiang Yu learned of this, he flew into a thunderous rage and immediately led his elite troops back to rescue Pengcheng. As a result, Liu Bang's Han army was utterly defeated by the Chu forces, suffering heavy casualties.
After fleeing to Xiayi (west of present-day Dangshan, Anhui), Liu Bang finally caught his breath. During a rest, he fumed to Zhang Liang, "This defeat has cost us dearly, and morale is shattered. I'm willing to offer all the land east of Hangu Pass as rewards to anyone who can help me crush Xiang Yu and avenge this humiliation. What do you think?"
Zhang Liang replied, 'The King of Jiujiang, Qing Bu, fights with great ferocity—though he serves Chu, he has long clashed with Xiang Yu. The general Peng Yue, commanding over ten thousand troops, has just raised the banner of rebellion against Chu. Your Majesty should send envoys to ally with these two forces. Among your own generals, only Han Xin can handle a major campaign on his own. If you grant them the lands east of Hangu Pass as their fiefs and have them throw their full strength against Xiang Yu, his defeat is certain!'
Following Zhang Liang's advice, Liu Bang sent envoys to ally with Qing Bu and Peng Yue, coordinating a joint assault on Xiang Yu. He also promoted Han Xin, dispatching him north of the Yellow River to open a new front and soon naming him the King of Qi. Han Xin lived up to Liu Bang's expectations, winning several consecutive victories against Xiang Yu's forces.
With the aid of Han Xin, Qing Bu, and Peng Yue, along with Xiao He securing a stable rear base and Zhang Liang offering strategic counsel, Liu Bang ultimately triumphed in the Chu-Han War and founded the Han Dynasty.
Later, the idiom "Taking Charge Alone" came to describe someone independently responsible for a particular area of work.
Source: *Book of Han*, "Biography of Zhang Liang"
Meaning of the Idiom: Later, the Chinese idiom "独当一面" came to describe someone independently responsible for a particular area of work.