The Awanosu Incident occurred on 10/8 of Tenshō 13 (1585) during which Nihonmatsu Yoshitsugu (Hatakeyama Yoshitsugu) abducted Date Terumune (a sengoku daimyō and the sixteenth head of the Date clan) after a meeting at Miyamori Castle in Nihonmatsu in Mutsu Province.  The event of this abduction is known as the Awanosu Incident after the placename where Terumune was later killed.

Prelude

In the tenth month of 1584, following the retirement of Terumune, his lineal heir, Date Masamune became the seventeenth head of the Date clan. 

In an effort to liberate himself from control by the Tamura clan, Ōuchi Sadatsuna, the lord of Obama Castle, joined forces with Nihonmatsu Yoshitsugu, the lord of Nihonmatsu Castle.  The Ōuchi requested support from the Ashina clan while the Tamura requested support from the Date.  Under these conditions, Ashina Moritaka and Yoshitsugu proposed to Terumune and Masamune a reconciliation between the Tamura and the Ōuchi.  Meanwhile, prior to the succession to Masamune, Terumune was engaged in diplomacy with the Ashina, attempting to mediate a settlement between the Ashina, the Iwaki, and the Tamura.  As a son-in-law of the Tamura, Masamune refused and, after the demise of Moritaka, the Ashina would not agree.  A failure to reach a settlement between the Date and Tamura on one side and the Ashina and Ōuchi on the other brought to an end a long-running alliance between the Date and Ashina clans.

In the fifth month of 1585, Masamune attacked Hinohara in the Ashina territory, and, in the eighth month, advanced his forces to Odemori Castle in the Ōuchi territory in the Adachi District of Mutsu Province.  At this time, Kikuchi Akitsuna served as lord of the castle.  On 8/27 of Tenshō 13 (1585), Masamune launched an all-out assault against the castle.  Standing on the front lines, he had his army with 8,000 arquebuses fire volleys upon the defenders, toppling the castle in the course of one day of violence.  As a warning to other would-be opponents, Masamune had the Date forces slaughter everyone beginning with Akitsuna along with the commanders, the soldiers, and even the women and children in the castle.  In a letter on this same date sent to his uncle, Mogami Yoshiaki, the lord of Yamagata Castle, Masamune noted that his forces made a clean sweep including Kikuchi Akitsuna and relatives of Ōuchi Sadatsuna, everyone else inside the castle, and even their dogs.  Altogether, this amounted to the killing of over 1,000 persons.  Nevertheless, an accurate number of those killed is unknown.  A letter dated on 8/28 sent to a retainer named Gotō Nobuyasu noted 200 persons while a letter dated 9/2 addressed to Kosai Sōitsu of the Shifuku Temple noted 800 persons.  

From among those inside the castle, it appears that some were separated and surrendered.  This included members of illegitimate branches of the Ōuchi clan who had received recognition of their rights to their lands from Masamune or else served Masamune.  Without regard to the number, however, it is a fact that many of the occupants of the castle were slaughtered.  This had an intense impact on Sadatsuna and his followers in addition to daimyō and other residents of the area.  The clean sweep at Odemori Castle remained a topic of discussion into later eras.  Several years later, in 1589, Masamune defeated the Ashina clan (sengoku daimyō in the area of Aizu) at the Battle of Suriageahara.  This led to the end of the Ashina clan.  Soon after the battle, the Date soldiers composed a folk song known as Sansa-shigure to celebrate the victory.  In the aftermath of Odemori, however, this otherwise popular song was not sung for generations in the area of the Adachi District where Odemori Castle was located.

Upon the fall of Odemori Castle, Sadatsuna abandoned Obama Castle and fled to Nihonmatsu Castle, later going to Aizu for the protection of the Ashina.  On 9/26, Masamune entered Obama Castle. Owing to the marriage by Yoshitsugu’s son, Nihonmatsu Yoshitsuna, to one of Sadatsuna’s daughters, Yoshitsugu refused to oppose Sadatsuna, inciting the anger of Masamune.  Having seen this outcome, in the tenth month, Yoshitsugu proposed conditions of surrender to Masamune.  Through the mediation of Terumune and Date Sanemoto, he was compelled to convey the Nihonmatsu territory (with the exception of only five villages) to Masamune.  Based on the conditions of surrender, Yoshitsugu sought a final showdown but yielded to the views of his retainers who argued that serving the Date would allow to secure their rights to their landholdings.

In an earnest appeal to Sanemoto, Yoshitsugu noted: “I cooperated during the attack on the Sōma so I want you to represent me.”  Sanemoto, however, lay ill so could not attend the negotiations and communicated the message to Terumune instead.  Masamune could not permit assistance be given to Sadatsuna so after invading Nihonmatsu ordered Yoshitsugu to tender his eldest son, Kuniōmaru, as a hostage.  On 10/6, Yoshitsugu then paid a sudden visit to Terumune at Miyamori Castle and communicated his intention to surrender.  That same day, Terumune informed Masamune in Obama and Masamune delegated the negotiations to Date Shigezane.  Upon receiving a communication from Shigezane, Yoshitsugu responded that he would not disavow the instructions of Shigezane even if he had to commit seppuku.  On 10/7, Yoshitsugu visited the encampment of Shigezane in Obama, lit candles, and met with Shigezane.

Course of events

Early in the morning on 10/8 of Tenshō 13 (1585), Yoshitsugu sent a messenger to Shigezane and expressed a desire to meet with Terumune.  On the pretext of giving his thanks to Terumune for the mediation, he visited Terumune who was staying at Miyamori Castle.  After the meeting, when Terumune stepped outside to send-off Yoshitsugu at the entrance, Yoshitsugu and his retainers drew their swords and abducted him.  

Date Shigezane and Rusu Masakage (who attended the meeting) brought troops and attempted to surround the departing party from a distance.  At Takadabaru along the shoreline of the Abukuma River on the border of the territory of the Nihonmatsu, Yoshitsugu and over fifty Nihonmatsu troops under him were attacked and killed by the Date forces.

At the time of the abduction, Masamune was engaged in a falconry outing.  Upon hearing the news, he quickly returned and pursued Yoshitsugu.  Under an alternate theory from the above, Terumune and his abductors were all killed by arquebus fire in an ensuing clash with Masamune.  There is also an unsubstantiated theory that the men who were engaged in falconry were carrying arquebuses as part of a plan for Masamune to kill his father.  

That evening, Terumune’s remains were taken to Obama.  Yoshitsugu’s remains were desecrated by Masamune, tied together with a wisteria vine, and ruthlessly suspended in a tree.  

Portrayals of the incident whereby Yoshitsugu and Terumune were shot by Masamune himself first appeared in a military account of the Ouu region written over 100 years later by a physician in 1698.  In another account, after Masamune rushed to the scene, Yoshitsugu sensed his determination and then stabbed Terumune to death.

Aftermath

On 10/15, after the completion of memorial services, Masamune, in a war of vengeance for his father, joined with Tamura Kiyoaki and Sōma Yoshitane to attack Nihonmatsu Castle defended by Araki Moritsugu.  On 11/2, an allied army was formed for the purpose of reinforcing Nihonmatsu whereupon Masamune left an army behind to continue the siege of the castle and led elite forces south toward Motomiya Castle to counter the allied army.

On 11/17 of Tenshō 13 (1586), the Date army violently clashed near Setogawa in the Battle of Hitotoribashi in the Adachi District with allied forces comprised of 30,000 soldiers from clans in the southern Oushū region led by the Satake clan who had gathered to support the defense of Nihonmatsu Castle.  Outnumbered, the Date forces scattered in defeat while Masamune himself confronted a dangerous situation amid a hail of arrows and bullets.  Through the efforts of a senior retainer named Oniniwa Yoshinao, who sacrificed himself serving as the rear guard, Masamune was able to retreat and, the following day, owing to the withdrawal of the Satake army, narrowly escaped with his life.