Rebel army (賊軍)
Rebel army is a term opposed to a term "Imperial Army" which was made to deny legitimacy of some armies which appeared in the history of Japan. Rebel army indicated armies formed against the emperor's (Imperial court's) will. It had the same meaning as emperor's enemy and in this sense, it was used mainly after the end of Edo period/the Meiji Restoration.
After the establishment of the Meiji Government, the term "Rebel army" indicated antigovernment forces.
"Rebel army" at the end of the Edo period
At the end of the Edo period, the Choshu Domain and their followers, who resorted to drastic measures such as the rebellion at the Kinmon Gate, were first called a rebel army. That was because Emperor Komei supported the policy of the Edo bakufu (Japanese feudal government headed by a shogun). But after the death of Emperor Komei, authority power relationship was changed in the Imperial Palace, and the army of the Edo Bakufu which had been treated as the Imperial army, but they were ousted from the position by conspiracy of the Satsuma and Choshu Domains. Since the army of the Satsuma and Choshu Domains became Imperial armies, the army of Bakufu who fought against Satsuma and Choshu became a "rebel army".
The army of The Edo Bakufu had been stated as a rebel army in history books for long time, and especially, the leading party of the Bakufu army, the Aizu Domain and the Ouetsu-reppan alliance were treated as the evil personified.
"Rebel army" during the Meiji period
During the Meiji period, only feudal retainers of the former Satsuma Domain and their followers were virtually treated as a "rebel army". Enemies such as the Qing Dynasty during the First Sino-Japanese War and the Russian Empire during the Japanese-Russo War were not called a "rebel army". Uprisings which frequently occurred at the early Meiji period were not treated as a "rebel army".
The army of the former Edo Bakufu had been disgraceduntil the postwar period, but people concerned in the Seinan War promptly took an effort to redeem their honor. Firstly, Takamori SAIGO was granted an amnesty in 1889, and then, until the end of the Taisho period, most people concerned in the Seinan War redeemed their honor.
If people win, they are regarded as a government army, but if they lose, they are regarded as a rebel army. Meaning: Whatever a process is like, everything is evaluated only by results.