The conspiracy was allegedly motivated by dissatisfaction with Mitsunari, who had written unfavorable assessments and underreported their achievements during the Imjin War against Korea and the Chinese Empire.

Mitsunari, however, learned of the plot through a report from Jiemon Kuwajima, a servant of Toyotomi Hideyori, and fled to Satake Yoshinobu's mansion with Shima Sakon and others to hide.

Ieyasu then negotiated a resolution, agreeing to allow Mitsunari to retire and to review the assessment of the Battle of Ulsan Castle in Korea, which had been a major point of contention.

[8] However, historian Watanabe Daimon, based on primary and secondary sources, suggests that the incident was more of a legal conflict between the generals and Mitsunari rather than a conspiracy to murder him.

[6][10] Muramatsu Shunkichi, author of The Surprising Colors and Desires of the Heroes of Japanese History and Violent Women, assessed that Mitsunari's failure in his conflict with Ieyasu was largely due to his unpopularity among major political figures of the time.

[11] On August 21, 1600, the Eastern Army alliance, which supported Tokugawa Ieyasu, attacked Takegahana Castle, defended by Oda Hidenobu, who sided with Mitsunari's faction.

[12] The army was divided into two groups: 18,000 soldiers led by Ikeda Terumasa and Asano Yoshinaga moved towards the river crossing, while 16,000 soldiers led by Masanori, Ii Naomasa, Hosokawa Tadaoki, Kyogoku Kochi, Kuroda Nagamasa, Katō Yoshiaki, Tōdō Takatora, Tanaka Yoshimasa, and Honda Tadakatsu proceeded downstream to Ichinomiya.

[13] The first group, led by Terumasa, crossed the Kiso River and engaged in battle at Yoneno, causing Hidenobu's forces to rout.

On August 22, at 9:00 AM, the Eastern Army led by Naomasa and Fukushima crossed the river and launched a direct attack on Takegahana Castle.

During the battle, Hidenobu's forces were deprived of expected support from Ishikawa Sadakiyo (石川貞清), who had decided not to assist the Western Army after reaching an agreement with Naomasa.

Hidenobu was prepared to commit seppuku but was persuaded by Ikeda Terumasa and others to surrender to the Eastern forces, leading to the fall of Gifu Castle.

Although Ukita's troops initially gained the upper hand, pushing back Masanori's forces, the tide turned when Kobayakawa Hideaki switched sides to support the Eastern Army.

Shortly after Ieyasu's death in 1619, Masanori was accused of violating the Buke Shohatto by repairing a small part of Hiroshima Castle, which had been damaged by a flood caused by a typhoon, without proper authorization.