Collapse at Moriyama「森山崩れ」

Matsudaira Clan

Owari Province

Matsudaira Kiyoyasu

The Collapse at Moriyama (Moriyama kuzure) occurred in the early morning on 12/5 of Tenbun 4 (1535).  In this incident, Matsudaira Kiyoyasu, the lord of Okazaki Castle and seventh head of the Matsudaira clan of Mikawa Province, was slayed by one of his retainers, Abe Masatoyo, while on a deployment in Moriyama in the Kasugai District of Owari Province.  Masatoyo committed the act on the mistaken belief that his father, Abe Sadayoshi, had been executed based on rumors of traitorous intent.


The Matsudaira clan extended its influence while serving the Ise clan.  In the spring of 1493, Ise Sadamune, who served as the secretary of a high-level organ of the Muromachi bakufu known as the mandokoro, collaborated with Hosokawa Masamoto, the deputy shōgun, to initiate a coup d’état known as the Meiō Political Incident (Meiō no seihen).  This event resulted in the ouster of Ashikaga Yoshiki (later known as Yoshitō then Yoshitane), the tenth shōgun, who fled to the Ōuchi clan in Suō Province for protection.  With the support of Masamoto, Ashikaga Yoshizumi became the eleventh shōgun.  Masamoto then aligned with Hosokawa Shigeyuki, the military governor of Mikawa, and garnered further support from Shiba Yoshitatsu, the military governor of Owari and Tōtōmi provinces.  Meanwhile, Imagawa Ujichika, a longtime rival of the Shiba clan who aspired to become the military governor of Tōtōmi, approached Ashikaga Yoshiki.  In this context, the Matsudaira clan entered into a period of confrontation with clans, such as the Kira, who supported the neighboring Yoshiki faction.

Following the assassination of Hosokawa Masamoto by retainers in the Lord Hosokawa Incident (Hosokawa-dono no hen) in the sixth month of 1507, a prolonged succession struggle ensued between his adopted sons, Hosokawa Sumimoto and Hosokawa Takakuni, with each aligned with separate members of the ruling Ashikaga family.  This broader struggle is known as the Eishō Disturbance (Eishō no sakuran), with the military component referred to as the Conflict between the Hosokawa (Ryō-Hosokawa no ran).  This conflict consumed the attention and resources of the Keichō-Hosokawa family (the main branch of the Hosokawa clan) for over two decades, until the defeat of Takakuni in a surprise attack by Sumimoto’s son, Hosokawa Harumoto, in a clash known as the Collapse at Daimotsu (Daimotsu kuzure).  Consequently, the Matsudaira could not count on the availability of reinforcements from the Hosokawa for other engagements during this period, and the Matsudaira lost to Imagawa Ujichika at the Eishō Mikawa Conflict.  Thereafter, Matsudaira Nobutada was forced into retirement by his father and former head of the clan, Matsudaira Nagachika, and other members of the family for failing to earn their trust.

Ōuchi Yoshioki, the sengoku daimyō based in Yamaguchi in Suō Province, viewed the succession struggle within the Hosokawa clan as an opportunity to assert control so, in the summer of 1508, he accompanied Ashikaga Yoshitō (formerly known as Yoshiki) in a march upon the capital of Kyōto.  Hosokawa Takakuni acted in concert with them, leading his rival, Hosokawa Sumimoto, along with Ashikaga Yoshizumi, to flee the capital.  Years later, after Yoshioki returned to his home province, Ashikaga Yoshitane (formerly known as Yoshitō) cut ties with Takakuni in favor of Sumimoto.  Takakuni then supported Yoshizumi’s son, Ashikaga Yoshiharu, who was opposed by Ashikaga Yoshitsuna, along with Sumimoto’s son and successor, Hosokawa Harumoto, based in Awa Province in Shikoku.  During this period, the Imagawa aligned with the faction supporting Yoshitane and Yoshitsuna, while the Matsudaira opposed the Imagawa by favoring Yoshizumi and his son, Yoshiharu.

Matsudaira Kiyoyasu succeeded his father, Matsudaira Nobutada, as head of the clan.  To govern the entire province of Mikawa and to oppose the Imagawa who had consolidated control of Tōtōmi and Suruga provinces, Kiyoyasu attacked Matsudaira Masayasu of the Okazaki-Matsudaira family who had sent his daughter to wed Mizuno Tadamasa, a supporter of Ashikaga Yoshitane, actively pursuing a policy to expand his domain.  After pressuring Shiba Yoshitatsu into retirement, Oda Tatsukatsu joined the faction supporting Yoshitsuna, causing Kiyoyasu to be opposed to both the Imagawa and the Oda.  He considered the dispatch of troops to Owari to take advantage of an opportunity after Imagawa Ujiteru became the tenth head of the Imagawa clan at the youthful age of fourteen.  As a result of this deployment, he became close to Tatsukatsu and received the daughter of Oda Nobusada as his formal wife, while his relationship with Matsudaira Nobusada who was not a lineal member of the main branch of the family deteriorated.

In 1535, Kiyoyasu joined with Oda Nobumitsu, the younger brother of Oda Nobuhide and lord of Moriyama Castle in Owari, deploying to Moriyama.  Nobumitsu had secretly colluded with Kiyoyasu, as well as retainers of the Saitō known as the Mino Group of Three (Mino sanninshū) in opposition to Nobuhide.  Kiyoyasu embarked on this deployment despite the objections of his elder retainers.  In the early morning on a day toward the end of the year, a disturbance arose on the base from an errant horse.  One of his retainers named Abe Masatoyo mistakenly concluded that his father, Abe Sadayoshi, had been executed by Kiyoyasu, so he struck down Kiyoyasu from behind, whereupon Masatoyo was then immediately killed.  Having lost their lord, the Matsudaira army retreated to Okazaki.  This killing of Kiyoyasu is known as the Collapse at Moriyama.


There was a rumor that Abe Sadayoshi was associated with recent betrayals to the Oda. Kiyoyasu apparently did not believe this rumor, but many other retainers viewed Sadayoshi with suspicion.  As a result, Sadayoshi summoned his eldest son, Abe Masatoyo, and told him that if he was killed on the basis of false accusations, then show their lord his written oath as evidence of his innocence, whereupon he passed the letter to his son.

Matsudaira Nobusada (Kiyoyasu’s uncle from the Sakurai-Matsudaira family) was said to have spread the rumor.  He became a relative by later arranging for the younger sister of Oda Nobuhide to wed his eldest son, Kiyosada, but Nobusada was not deployed to Moriyama at the time of the incident.  When Kiyoyasu was in Anjō, Nobusada reproached one of his retainers named Ochiai Kahei-ei, and Kahei-ei’s gave a splendid response, prompting Kiyoyasu to increase his stipend by 500 kan mon.

In the sixth month of 1537, perhaps owing to intervention by the Imagawa clan as sengoku daimyō, and the Kira clan, Nobusada departed Okazaki Castle and returned to Sakurai Castle.  In later years, he plead to Hirotada for forgiveness.

After the incident

Kiyoyasu was succeeded by his eldest son, Matsudaira Hirotada, who was around ten years of age at the time of his father’s unexpected demise.  This created an opportunity for Oda Nobuhide to attack the Matsudaira.  Meanwhile, Nobusada, the grand-uncle of Hirotada, exploited the situation by taking decisive command of Okazaki.  Hirotada’s great-grandfather, Matsudaira Nagachika, resided in Okazaki at the time but did not protest.  Instead, he implicitly accepted Nobusada’s actions, thereby enabling Nobusada to acquire more authority.  Later that year, Nobusada seized the former territory of Kiyoyasu, solicited families who had supported the Matsudaira for generations, and plotted to assassinate Hirotada.  Owing to efforts by Abe Sadayoshi, and protection by Kira Mochihiro, Hirotada managed to escape to Ise Province and then Kobe where he went underground.

In 1536, a succession struggle broke out within the Imagawa clan known as the Hanakura Conflict (Hanakura no ran).  This was triggered by the death of Imagawa Ujiteru, the tenth head of the Imagawa clan, while en route to a gather of tanka poets in Odawara.   Ujiteru may have been poisoned or assassinated, but the reasons for his death remain uncertain.  In the ensuing struggle, Sengaku Shōhō (later known as Imagawa Yoshimoto) competed against Genkō Etan to become the successor to Ujiteru as head of the clan.  Shōhō was seventeen years old at the time, having first entered into temple life at the age of four under the tutelage of Taigen Sessai at the Zentoku Temple in Suruga Province.  Upon returning to secular life, Shōhō changed his name to Imagawa Yoshimoto, receiving one of the characters in his name from Ashikaga Yoshiharu, the shōgun.  Yoshimoto reconciled with the Takeda and became a supporter of Yoshiharu, changing the diplomatic policies of the Imagawa (forging an alliance with the Takeda of Kai Province and breaking an alliance with the Hōjō of Sagami Province).

Kira Mochihiro of the Tōjō-Kira clan supported Matsudaira Hirotada, both of whom were aligned with the Yoshiharu faction.  With the support of the Imagawa and the Tōjō-Kira clans, Hirotada succeeded in recapturing the former base of his father, Matsudaira Kiyoyasu, at Okazaki Castle, while Nobusada returned to Sakurai Castle.  Then, Imagawa Ujiteru, lord of Nagoya Castle and an advocate of Yoshiharu, incurred an attack by Oda Nobuhide, heightening concerns with respect to the rising threat posed by the Oda.  Although the Matsudaira clan had enabling the return of Hirotada to Okazaki, a power-struggle broke out between the only uncle with whom he had a blood relationship, Matsudaira Nobutaka, and a band of retainers from the main branch of the clan.  This caused Nobutaka to turn to the Oda for support.

Acting upon the direction of Nobusada, Sakai Saemon-no-jō, Ōhara Sakon-uemon, and Imamura Denjirō urged Hirotada to have two of his retainers, Ishikawa Kiyokane and Sakai Masachika, commit seppuku, but Hirotada did not comply.  Hirotada became opposed to Matsudaira Tadamichi and Matsudaira Kiyosada (Nobusada’s son) after they began to collude with the Oda, followed by Sakai Saemon-no-jō, Ōhara Sakon-uemon, and Imamura Denjirō.  Meanwhile, the Toda clan of the Atsumi District and the Makino clan of the Hoi District of Mikawa received support from the Oda and began again to show signs of autonomy.  The Matsudaira could not respond to this on their own, so obtained additional forces by pledging Takechiyo (later Tokugawa Ieyasu) as a hostage to the Imagawa clan.

In 1566, thirty years after the death of Kiyoyasu, the Matsudaira finally reacquired control over Mikawa Province in the era of his grandson, Ieyasu.

Enigmas surrounding the event

There are numerous uncertainties regarding this event.  One of these is the treatment of Abe Sadayoshi, the father of the assailant, after the event.  Not only was Sadayoshi not subject to recriminations, he was assigned to lead a group of soldiers from Mikawa as a retainer of Hirotada, the son of Matsudaira Kiyoyasu, the victim of the slaying.  Based on the norms of the times, he would have been executed for being associated with the crime.  Even if not executed, he would ordinarily have been reproached for the act.  Under one theory, Sadayoshi attempted to kill himself out of a sense of responsibility for the crime, but after Hirotada intervened, he served as Hirotada’s retainer.  After Abe Masatoyo struck down Kiyoyasu, Uemura Ujiaki beheaded him.  In 1549, Iwamatsu Hachiya stabbed Hirotada with a short sword and killed him. Once again, Uemura Ujiaki decapitated the assailant of his lord, an act for which he received a letter of commendation.  Based on certain theories, it is not coincidental that two generations of clan leaders were assassinated and then the same individual issued the immediate punishment; however, from the start, there have been various theories as to Hirotada’s cause of death.  There are also multiple theories as to the circumstances of the assault by Iwamatsu Hachiya against Hirotada.