During the 1300s, there were two competing courts in Japan. The consequences of events in this period continue to be influential in modern Japan's conventional view of the Tennō Seika (Emperor system).
This link can help to understand the Nanbokucho Period.
https://doyouknowjapan.com/history/nanbokucho/
- The Southern Court (南朝 Nanchō) were a set of four emperors (Emperor Go-Daigo and his line) whose claims to sovereignty during the Nanboku-chō period spanning from 1336 through 1392 were usurped by the Northern Court. This period ended with the Southern Court definitively losing the war, and they were forced to completely submit sovereignty to the Northern Court.
- The Northern Court (北朝 hokuchō), also known as the Ashikaga Pretenders or Northern Pretenders, were a set of six pretenders to the throne of Japan during the Nanboku-chō period from 1336 through 1392. The present Japanese Imperial Family is descended of the Northern Court
Since the 19th century the Emperors of the Southern Imperial Court have been considered the legitimate Emperors of Japan. Other contributing factors were the Southern Court's control of the Japanese imperial regalia, and Kitabatake Chikafusa's work Jinnō Shōtōki, which legitimized the South's imperial court despite their
Anyway, what made the determination was the possession of the three sacred regalias of Japan, which the Southern court had. The southern court’s last Emperor, Go-Kameyama, gave them to the Northern court with the understanding that the two lineages would alternate in succession. However, the Muromachi Bakufu reneged on the agreement.
Under the government of Prime Minister Katsura Tarō and under the influence of State Shinto, an Imperial decree dated March 3, 1911, established that the legitimate reigning monarchs of this period were the Southern Court, there was no adverse effect for the imperial dynasty.
In OTL, one Southern Court descendant, Kumazawa Hiromichi, declared himself to be Japan's rightful Emperor in the days after the end of the Pacific War. He claimed that Emperor Hirohito was a fraud, arguing that Hirohito's entire line is descended from the Northern Court. Despite this, he was not arrested for lèse-majesté, even when donning the Imperial Crest. He could and did produce a koseki detailing his bloodline back to Go-Daigo in Yoshino, but his claims and rhetoric failed to inspire anything other than sympathy.
Could the 1911 decree have led to a civil war in Japan after the death of Emperor Meiji in 1912?
I think of this specific possibility due to the fragile health of his successor, Emperor Taishō , who suffered from various neurological.
It could also happen after the 1923 Great Kantō Earthquake, often interpreted as an act of divine punishment to admonish the Japanese people for their self-centered, immoral, and extravagant lifestyles.