Beginning of hostilities (931–936)
Disagreement exists about the exact cause of Masakado's rebellion. While some sources portray the uprising as revenge for his failure to secure a government post, the Shōmonki (将門記; also read as Masakadoki), an anonymous monograph on Masakado's life believed to have been completed as early as the 940s, suggests that the conflict originally began in 931 (Jōhei 1) as a dispute between Masakado, freshly returned from Heian-kyō, and his paternal uncle Taira no Yoshikane (平良兼) over a woman. The identity of this woman is uncertain, though one theory suggests that it may have been a daughter of Yoshikane who married her cousin and apparently went to live with him against her father's wishes. (Aristocratic marriages during the Heian period were usually matrilocal; the wife continued to reside with her parents while the husband either moved in with his wife's family or simply visited her.) Besides this affront to his honor, Masakado not obtaining any post or rank in the capital might have been another factor in Yoshikane's opposition to the marriage. Another theory based on folk tradition meanwhile suggests that Masakado and Yoshikane quarreled over a daughter of Minamoto no Mamoru (源護), former senior secretary (大掾, daijō) of Hitachi Province who had married off his daughters to Masakado's uncles, Yoshikane among them.
On the other hand, the Konjaku Monogatarishū (c. 1120) gives another reason for the conflict, namely that Masakado's uncles had appropriated the lands the young man was supposed to inherit from his late father. Masakado's uncle Taira no Kunika (平国香), who as Takamochi's eldest son was the head of the clan, might have tried to take over his younger brother Yoshimochi's property and place it under his control. Kunika, like Yoshikane, was related by marriage ties to Mamoru, who would eventually become involved in the conflict.
In the 2nd month of 935 (Jōhei 5), Masakado and his men were ambushed by Mamoru's three sons, Tasuku, Takashi, and Shigeru, at a place called Nomoto (野本) in the district of Makabe (真壁郡), near the border between Hitachi and Shimōsa (modern Chikusei, Ibaraki), but managed to repel their attack; the three brothers all died in the battle. In retaliation, Masakado then burned and ransacked the houses of Tasuku's supporters across southwestern Hitachi. Kunika also died during this conflict, under circumstances not entirely clear: he may have either been killed during the skirmish at Nomoto or when Masakado set fire to his residence.
On the 21st day of the 10th month of the same year, Taira no Yoshimasa (平良正), Masakado's paternal uncle or cousin who was also related by marriage to Mamoru, seeking to avenge the deaths of Kunika and Mamoru's sons, faced Masakado in battle in the village of Kawawa (川曲村) in western Hitachi (identified with the town of Yachiyo, Ibaraki) but Masakado once again proved the victor; more than sixty of Yoshikane's men were killed while the rest dispersed.
After his humiliating defeat, Yoshimasa called to Yoshikane – now the vice-governor of Kazusa Province – for aid, who then gathered a large number of warriors from Kazusa and Shimōsa such that officials from the two provinces initially attempted to prevent their dispatch (Such protests were later withdrawn after the issue was deemed to be a private matter beyond the sphere of state affairs). On the 26th day of the 6th month of 936 (Jōhei 6), Yoshikane led his massive army to Hitachi, where he joined forces with Yoshimasa and Kunika's son Sadamori (who had been in the capital when his father was killed and initially took a neutral stance), whom he had prevailed upon to take part in the attack against Masakado. They made contact at the border between Hitachi and Shimotsuke (modern Tochigi Prefecture) with Masakado, who went there to verify reports about a plan to launch a joint attack on him from the north. Despite only having about a hundred poorly-equipped soldiers with him, Masakado inflicted heavy casualties upon his enemies' several thousand strong army. Scattered and thrown into confusion, Yoshikane and the remnants of his forces fled to the provincial headquarters of Shimotsuke, Masakado pursuing them. Although he managed to surround his uncle in the governmental offices, Masakado, seemingly concerned about subsequent censure should he kill Yoshikane then and there, allowed him to escape through a gap in his western line. He then filed a formal grievance with the provincial authorities in neighboring provinces before returning to his territory.
Not long after his victory, Masakado received a summons from the imperial court because of a complaint lodged against him by Minamoto no Mamoru over the battle at Nomoto. Masakado then hurried to the capital to give an account of himself; his lord, Fujiwara no Tadahira, probably intervened in the case and helped lighten his punishment. He was eventually pardoned early the following year (937 / Jōhei 7) when a general amnesty was declared at the occasion of Emperor Suzaku's coming of age (genpuku).
