The Chōroku Incident (Chōroku no hen) occurred on 12/2 of Chōroku 1 (1457). In this incident, remnants of the Akamatsu clan attacked the provisional palace of leaders of the gonanchō, or post-Southern Court, killing Emperor Takahide and Emperor Tadayoshi, brothers of Imperial descent who had aimed for independence from the unification of the Northern and Southern Courts that occurred in 1392. Akamatsu Masanori was appointed military governor of one-half of Kaga Province owing to his contributions during the Chōroku Incident to recover the Yasakani-no-Magatama, one of the three sacred Imperial Regalia of Japan.
The sacred regalia were comprised of the Yasakani-no-Magatama (a curve-shaped jewel), the Yata-no-Kagami (a mirror), and the Kusanagi-no-tsurugi (a sword). These items possessed mystical significance. According to Japanese mythology, the jewel had been used, along with the mirror, to lure the sun goddess Amaterasu from a cave. These items had originally been handed down to Japanese Emperors by Amaterasu through her grandson Ninigi-no-mikoto. As Imperial Regalia, the virtue of the Yasakani-no-Magatama is gentleness and yielding, and a source of compassion. The courage of the sword and the wisdom of the mirror were tempered by the jewel’s call for compassion from the Emperor of Japan.
In the ninth month of 1443, at the Kinketsu Incident (Kinketsu no hen), the leaders of the movement to restore the Southern Court took the Yasakani-no-Magatama. This loss remained a lingering issue for remnants of the Akamatsu including Kōzuki Mitsuyoshi, Iwami Tarō, and Nibutani Tatewaki Saemon and Shirō Saemon (brothers) who aimed to revive the clan after the loss of their status as military governors during the Kakitsu Disturbance (Kakitsu no ran). The Yamana clan occupied the former territory of the Akamatsu, while retainers of the Akamatsu were banished from their homes and left to wander the provinces. These dispirited men aspired to revive their clan. Moreover, the Imperial court and the bakufu promised the clan could be restored upon recovery of the Yasakani-no-Magatama from the leaders of the independence movement.
The remnants of the Akamatsu devised a scheme to approach those in possession of the regalia, informing the post-Southern Court that, since the Kakitsu Disturbance, their surviving members had no lord, and could no longer tolerate their circumstances, so they desired a visit to the Court in Yoshino in a bid to recapture the capital. The leaders of the post-Southern Court accepted this proposal, and, on 12/20 of 1456, thirty members from the Akamatsu headed toward Yoshino.
After entering Yoshino, the Akamatsu did not immediately take action. First, they searched for the location of the Yasakani-no-Magatama. After approximately one year, the Akamatsu determined that the regalia were located at a provisional palace of the post-Southern Court on a northern mountain in Oku-Yoshino. Around midnight on 12/2 of 1457, the Akamatsu split into two groups and launched attacks against Emperor Takahide (at Kitayama, a shrine of the first rank) and Emperor Tadayoshi (at Kōno, a shrine of the second rank). These brothers of Imperial descent led the independence movement that served as the basis for the post-Southern Court. Emperor Takahide was killed by the elder of the Nibutani brothers, who stole back the Yasakani-no-Magatama. Meanwhile, Emperor Tadayoshi was slayed at the Kōno Shrine by Kōzuki Mitsuyoshi. Other members of the post-Southern Court, including Nonagase Moritaka and Morizane (brothers), and Kusunoki Masamichi, supported Emperor Takamasa, and fled to the mountains in Oku-Yoshino, but were later killed near the Totsu River.
Although the Akamatsu succeeded in reclaiming the coveted Yasakani-no-Magatama, their attacks were witnessed by the citizens of Yoshino. A counterattack by these citizens led to the slaying of the Nibutani brothers in the Oba Valley. The Yasakani-no-Magatama, along with Emperor Takahide’s head, were recovered. Thereafter, the Akamatsu departed from Yoshino.
At the end of the third month of 1458, the remnants of the Akamatsu joined with Ogawa Hiromitsu (of a wealthy family from Yamato Province) and Ochi Iehide to launch a meticulously planned attack on the residence of Takahide’s mother in Yoshino to whom the Yasakani-no-Magatama had been returned. The attackers succeeded in reclaiming the jewel, whereupon, on 8/30, it was taken to Kyōto and given to the Imperial court.
The Muromachi bakufu recognized the success in recovering the Yasakani-no-Magatama that had been out of the capital for approximately fifteen years, permitting the Akamatsu to revive their clan under Akamatsu Masanori. Moreover, the clan was appointed to serve as the military governor of the northern half of Kaga Province, and given control of Nitta manor in Kōzuke Province and Takamiya District in Ise Province. Hosokawa Masamoto actively participated in the granting of territory and restoration of the Akamatsu clan. Masamoto may have promoted the Akamatsu for political reasons to serve as a counter to Yamana Sōzen.