Photo/Illutration The "kishomon" oath document Hashiba Hideyoshi sent to Uehara Motosuke before learning of Oda Nobunaga's assassination (Eiichi Miyashiro)

A document sent by samurai Toyotomi (then Hashiba) Hideyoshi just prior to learning of the death of his master, warlord Oda Nobunaga (1534-1582), has been discovered and purchased on an auction site.

Yuki Murai, an associate professor at the Historiographical Institute of the University of Tokyo who specializes in medieval Japanese history, announced on Nov. 20 that he found a “kishomon” (oath document) sent by Hideyoshi.

“It's a valuable historical resource that shows Hideyoshi’s actions just before receiving news of a major historical event,” Murai said.

The event is known as the Honnoji Incident, marking the assassination of Nobunaga on June 2, 1582. 

One day later, his vassal Hideyoshi, who was fighting the Mori clan in the Chugoku region, sent a kishomon to a powerful Mori-affiliated commander who had defected, promising him a province as a reward. 

The document measures approximately 33 centimeters in height and 45 cm in width.

The contents appear to be addressed to Uehara Motosuke, a samurai of Bingo province (now eastern Hiroshima Prefecture) who was related by marriage to the Mori clan. It is dated June 3, Tensho 10 (1582).

“Since you have pledged loyalty to lord Nobunaga, I will obtain and send you a 'shuinjo' (vermilion-sealed letter) from him granting you rights to Bingo province,” it said.

At the time, Hideyoshi was besieging Shimizu Muneharu, a military commander, who was holed up in Bitchu Takamatsu Castle.

It was considered significant that the Uehara clan, stationed at nearby Hibata Castle, switched sides and became an ally of Hideyoshi.

It is believed that Hideyoshi learned of the Honnoji Incident later that night or early the next morning.

“This is the last document Hideyoshi issued under the belief that his lord Nobunaga was still alive. Promising to grant Bingo, which he had not yet secured, shows Hideyoshi’s bold character and is interesting,” Murai said.

The document will be displayed at a special exhibition starting Feb. 7 next year at the Ibaraki Prefectural Archives and History Museum, located in Mito.

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