When Xiang Yu's uncle, Xiang Liang, raised an army to rebel against the Qin dynasty, his strategist Fan Zeng advised him: "The descendants of the six warring states are each raising troops to restore their kingdoms, but without unified command, the Qin forces can crush them one by one, repeating the old disaster. I hear that the grandson of King Huai of Chu is living in obscurity, herding sheep for a family. Why not install him as king and use his name to rally the realm?" Xiang Liang agreed and made Huai's grandson the new King of Chu.
After several victories, Xiang Liang grew overconfident and underestimated the Qin army
Xiang Yu led his army to attack the Qin forces, crushing their main army. King Huai of Chu made a pact with the generals: whoever destroyed Qin first would become the King of Guanzhong. While Xiang Yu fought in the north, Liu Bang entered the pass first and conquered Qin. After Xiang Yu's victory in the north, he marched into the pass with 400,000 troops, killed the Qin emperor Ziying, and set fire to the Qin palaces, with flames raging for three months.
Xiang Yu, believing he had secured the realm, granted King Chu the empty title of "Righteous Emperor," styled himself Hegemon-King of Western Chu, enfeoffed Liu Bang as King of Han in the treacherous Ba and Shu regions to hinder his future expansion, and appointed descendants of the six other states and surrendered Qin generals as King of Western Wei, King of Henan, King of Yong, and so on.
Before Xiang Yu plundered the Qin palace's women and treasures and returned east to Pengcheng, he said to the Righteous Emperor, "Ancient emperors ruled over lands a thousand li in extent. Now, south of Changsha Commandery, west of Jiuyi Mountain, in Chen County on the upper reaches of the Xiang River, there remains a territory yet unassigned. Go there."
When Emperor Yi planned to travel south through Jiangling, Xiang Yu secretly ordered the local Prince of Linjiang to escort him under the guise of protection, then assassinate the emperor aboard the boat during the river crossing.
After defeating the three kings of Guanzhong—Yong, Sai, and Di—near Hanzhong and annexing their lands, Liu Bang, the King of Han, distributed wealth and farmland to the local people, reduced forced labor and taxes, and established the Han Dynasty.
After Xiang Yu's new enfeoffments outside the pass, the feudal lords, deeming the distribution unfair, began raising armies and fighting each other, with the kings warring among themselves, and Xiang Yu also joining the fray against them.
Taking advantage of the chaos beyond the pass, the King of Han marched east from Xianyang, his army pressing directly toward Luoyang.
A local official named Dong, who oversaw education, knelt before Liu Bang and advised, "Those who follow virtue prosper, while those who defy it perish. An army without a just cause cannot achieve great things—only by declaring a campaign against rebels can you win the people's support. Xiang Yu murdered his own sovereign, Emperor Yi, making him the realm's chief villain. A righteous army cannot be stopped by any foe, no matter how fierce. I beg you, my lord, order the troops to wear mourning for Emperor Yi and announce to all lords that this eastern expedition is to punish the traitor. The whole land will rally to you; though Xiang Yu commands many soldiers and brave generals, he will only meet defeat."
Liu Bang said, "I thank you for your teachings, Master." He immediately ordered a funeral for the Righteous Emperor, personally attended the altar to weep bitterly, and issued a proclamation to the feudal lords, saying: "The world jointly established the Righteous Emperor as sovereign. Now Xiang Yu has murdered the emperor south of the Yangtze—an act of utter treason and inhumanity. I shall raise the full forces of the realm to join with all lords and princes in capturing the murderer of the emperor."
After four or five grueling years of bitter warfare, King Liu Bang of Han, with righteous justification, led his army out of the pass and finally conquered the realm, defeating Xiang Yu.
"Launching troops without justification" means sending an army without a legitimate reason; later, it also broadly refers to doing something without proper cause.
Source: *Book of Han*, Chapter "Annals of Emperor Gao, Part Two"
Meaning of the Idiom: Later, the Chinese idiom "兵出无名" came to describe how sending an army without a legitimate reason later also broadly refers to doing something without proper cause.