The Shimotsuki Incident (霜月騒動)

The Simotsuki Incident was a political change in the Kamakura bakufu (Japanese feudal government headed by a shogun) that occurred in Kamakura in the latter half of the Kamakura period on December 21, 1285. After the death of the eighth regent Tokimune HOJO, the senior vassal Yasumori ADACHI and TAIRA no Yoritsune, Uchi-Kanrei (head of Tokuso Family [the patrimonial head of the main branch of the Hojo clan]) opposed each other and Yasumori, his family, and his party were destroyed in this incident in which Yoritsuna's party made the preemptive attack. This incident is also called the Battle of Koan, the Yasumori ADACHI war or the Jonosuke AKITA war.

Background

Yasumori ADACHI was a member of the Adachi clan, the senior vassals since the Kamakura government was established, and he supported the regent Tokimune HOJO as a maternal relative of the Tokuso Family of the Hojo clan, and was a key figure experienced in important positions like an Ossobugyo (temporary position in charge of retrials and accepting appeals in Kamakura bakufu) and Goon bugyo (commissioner in charge of issuing the deed of rewards.)
TAIRA no Yoritsuna was a male nurse of Tokimune and the under secretary of the head of Tokuso family and he was in a position to embody the authority of Tokuso. In Kamakura bakufu, Yasumori who was supported by tozama gokenin (nonhereditary lower-ranking vassal in the Kamakura period) competed against Tokuso's vassal party led by Yoritsuna. In 1284, Tokimune, the regent to the shogunate who had played a coordinating role between both parties, died and his 14-year-old legitimate child Sadatoki HOJO became the ninth regent. Confrontation and rivalry became intense over government administration amid internal and external issues erupting one after another since the Mongol invasion attempts on Japan. After Yasumori got a position to lead the government administration as the maternal grandfather of Sadatoki, he launched a political reform called 'Koan tokusei' and promulgated a large number of laws and ordinances. Yasumori's reforms aimed to reorganize the Gokenin system (immediate vassals of the shogunate in the Kamakura period) overseen by Seitaishogun (commander-in-chief of the expeditionary force against the barbarians, great, unifying leader) in order to expand the Gokenin class and enhance the shogun's authority to prevent any increase in Tokuso's power and Gokenin's intervention into politics, which would influence the position of Yoritsuna and other Tokuso's vassals.

Progress

Yoritsuna requested Genkei, the betto (the head secretary of a temple) in Nikko-zan mountain to give an invocation to subdue Yasumori on December 8 and 18, 1285. According to the Shimotsuki Incident note, the only primary archive describing the incident, when Yasumori was in his second house in Matsutani in the morning of December 21, he noticed a restlessness in the world and went back to his residence in Tonotsuji around midday to prepare to attend court, but he was attacked and killed by Yoritsuna's soldiers when he arrived at Sadatoki's residence. This attack left 30 people dead and 10 people injured triggering a big conflict which resulted in fire to spread to the shogun's palace and the Adachi's party was defeated by 4 pm.

Tokikage ADACHI escaped to Iiyama, but he was killed there.
About 500 members of Yasumori's family committed suicide and the incident extended across country and Yasumori's party in various places were pursued and forced to commit suicide,
The damage to the Gokenin in Kozuke Province and Musashi Province, the Adachi clan's bases, was huge, for example, Shokyozaemon Muto (武藤少卿左衛門) in Musashi, Muneaki ADACHI (安達宗顕) in Totomo province, Shigekage ADACHI in Hitachi province and Hikojiro BANNO in Shinano province committed suicide. In Kyushu region, Morimune ADACHI, son of Yasumori died during the battle of Iwato. It is supposed that such a large number of people died in the battle or were forced to commit suicide because this attack was carried out simultaneously based on a well-prepared plan.

Victims of this battle among the members of Yasumori's party other than the Yasumori family were the Ogasawara clan, the Ashikaga clan, the Tomono clan, the Ito clan, and the Muto clan (Shoni clan) and many powerful gokenin since the foundation of Kamakura bakufu also suffered, such as Ainori FUJIWARA (藤原相範), Mitsuruuji KIRA, Matataro UETA (Yasuhiro Oe), Saburozaemon KOBAYAKAWA, Kurodo SANKA, Kagemura AMANO, Kageie IGA, Yukiie NIKAIDO, Mimasakasaburozaemon (OI) (美作三郎左衛門), Jironyudo TSUNASHIMA (綱島二郎入道), a lord of Tonaizaemon IKEGAMI (池上藤内左衛門), Shojiro YUKIKATA (行方少二郎), Magojiro (Masatsura?) NANBU, Saburo ARISAKA, Yatonisaemoni KAMATA (鎌田弥藤二左衛門尉), and Hitobito AKIYAMA (秋山人々). On the other hand, the Sasaki clan, the Imagawa clan and the Chiba clan joined this attack as pursuer of Tokuso's vassal.
This conflict split the bakufu into two large groups, even a family was broken down into two; Yoritsuna party and Yasumori party,
Sadaaki KANESAWA, Yasumori's relative, was confined in Shimousa Province and Kagetuna UTSUNOMIYA and Munehide NAGAI fell from power.

Kamakura bakufu after the Shimotsuki incident

TAIRA no Yoritsuna held a real power and rejected Yasumori's Koan reform, and the most of the positions in the bakufu were occupied by the Hojo family, with Tokuso at the top and the Osaragi line of the Hojo clan and the Nagoe line of the Hojo clan at the center, and no important positions were given to old gokenin like the Ashikaga clan.
In Kyoto, the retired Emperor Kameyama, who is said to have carried out Koan tokusei (political reforms in the Koan era) with Yasumori was forced to terminate his Insei (government by a retired emperor) (Jimyoin line Emperor Fushimi ascended the throne.)

Shogun issue

According to "Horyakukanki" (A History Book of the 14 century in Japan), Yoritsuna gave Sadatoki a slanderous account of Mumekage Adachi, a son of Yasumori, saying that he was claiming himself as an illegitimate child of MINAMOTO no Yoritomo and changed his family name to the Minamoto clan to become shogun by raising a rebellion. Yasumori found 'Higekiri' (celebrated sword) handed down in Genji shogun in some reisha shrine in Kyoto and dedicated it to the Buddhist altar in the Hokkedo hall.
Although the Higekiri sword was lost during the Shimotsuki incident, it was discovered on January 8, and Sadatoki indicated it was wrapped by "Red brocade" (Red was the flag color of the Hojo clan who refered themselves as the Taira clan.)

Originally, the Kamakura bakufu was established based on master-servant relationship between the Kamakura-dono (lord of Kamakura) and the Gokenin, however the Soryo system (the eldest son system for the succession of the head of the family) was getting bogged down. Gokenin faced financial issues and they were employed by Tokuso's vassals as a Miuchibito (private vassals of the tokuso) and a master-servant relationship was established between Tokuso and Miuchibito. Petty gokenins who had been the subject of relief in Yasumori's reform became pursuers of Tokuso party as Miuchibito. On the other hand, Gokenins since the foundation of Kamakura bakufu were offended by Uchi-Kanrei and Miuchibito since Uchi-kanreia holding the reins of power despite they were of lower ranks than Gokenin and, as a result, most of tozama gokenin (nonhereditary lower-ranking vassal in the Kamakura period) joined Yasumori's party. The Hojo clan couldn't get over Shogun issue in the end as Tokuso couldn't become Shogun due to his low status by birth eventhough he was the most powerful lord of the Kamakura bakufu.