After Nobunaga was killed in the Honnō-ji Incident in 1582, Nagamasa served Toyotomi Hideyoshi along with his father and participated in the invasion of Chūgoku.

[7] In 15 July, following the Battle of Imjin River, Nagamasa led his forces west into Hwanghae Province, where he participated in the first Siege of Pyongyang.

[10] Later on, Nagamasa launched a night raid using a crane formation pincer attack with the intention of crushing enemy forces from each end.

[13] During his tenure in the Korean campaign, there is a famous anecdote which is attributed to Katō Kiyomasa that he was hunting a tiger during his free time.

[6] According to popular belief, in 1598, after the death of Toyotomi Hideyoshi, the government of Japan had an incident when seven military generals consisting of Fukushima Masanori, Katō Kiyomasa, Ikeda Terumasa, Hosokawa Tadaoki, Asano Yoshinaga, Katō Yoshiaki, and Kuroda Nagamasa planned a conspiracy to kill Ishida Mitsunari.

However, when Mitsunari learned of this through a report from a servant of Toyotomi Hideyori named Jiemon Kuwajima, he fled to Satake Yoshinobu's mansion together with Shima Sakon and others to hide.

Ieyasu then negotiated a compromise to allow Mitsunari retire, as well as review the assessment of the Battle of Ulsan Castle in Korea, which was one of the main issues that led to the incident.

[17] However, historian Watanabe Daimon has also stated in gathering from primary and secondary source texts written about the accident, that this was more of legal conflict between those generals with Mitsunari, rather than a conspiracy to murder him.

[14][19] Muramatsu Shunkichi, writer of "The Surprising Colors and Desires of the Heroes of Japanese History and Violent Women”, gave his assessment that the reason for Mitsunari's failure in his war against Ieyasu was due to his unpopularity among the major political figures of that era.

On August 21st, The Eastern Army Alliance, who had sided with Ieyasu Tokugawa, attacked Takegahana castle which was defended by Oda Hidenobu, an ally of the Mitsunari faction.

[21] The Eastern Army split themselves into two groups, with 18,000 soldiers led by Ikeda Terumasa and Asano Yoshinaga dispatched to the river crossing, while 16,000 soldiers led by Nagamasa, Fukushima Masanori, Hosokawa Tadaoki, Kyogoku Kochi, Ii Naomasa, Katō Yoshiaki, Tōdō Takatora, Tanaka Yoshimasa, and Honda Tadakatsu headed downstream at Ichinomiya.

[22] The first group led by Terumasa crossed the Kiso River and engaged in a battle at Yoneno, routing Hidenobu forces.

The second Eastern Army group led by Nagamasa and others crossed the river and launched a direct attack on Takegahana Castle at 9:00 AM on August 22nd.

As a reward for his performance in the battle, Ieyasu granted Nagamasa Chikuzen [2] – 520.000 koku – in exchange for his previous fief of Nakatsu in Buzen.