Hideyoshi was so cautious around Yoshitaka that eventually he started looking to Ishida Mitsunari for counsel in his stead, and in reaction to this, Yoshitaka withdrew from life and took the name "Josui."

This brings us to Nagamasa, who thus became the head of the Kuroda house in 1589. Nagamasa had fought his first battle when he was 16 (1584), and was known for his ferocity and bravery in battle. When cautioned of fighting at the forefront of a battle, he stated, "Even if I die, as long as my father is alive the Kuroda house will have nothing to fear." [paraphrased]

Continuing this reputation during the Korean campaign, Nagamasa came back to Japan, and was faced with the decision of which army to support in the upcoming battle of Sekigahara. Despite the Kuroda family's close association with Hideyoshi (whom Ishida was now representing), he chose Ieyasu's army, for reasons which are not entirely known. It might have been that he was bitter at Hideyoshi for overlooking his father, it may have been animosity towards Mitsunari himself, or it may have simply been recognition that Ieyasu would succeed where Mitsunari would not.

In any case, on the day of the battle Nagamasa would be at the right flank of the vanguard of the eastern army, at the head of 6000 men. Taking part in a massed charge at Ishida's position, Nagamasa and his men would succeed in driving Mitsunari into a defensive posture. One of his men also succeeded in shooting down Shima Sakon. In response to his contributions, Ieyasu would later reward Nagamasa by giving him part of the fief of Chikuzen, worth 520,000 koku. Following this Nagamasa was a Tokugawa follower, even divorcing his wife (who had Toyotomi connections) and re-marrying to an adopted daughter of Ieyasu.

Yet despite this, Ieyasu seems to have been wary of Nagamasa, perhaps suspecting that he still had ties with the Toyotomi house. Denied participation in Fuyu-no-jin of 1614, he was finally allowed to fight in Natsu-no-jin with a token force. He died in 1623 at the age of 54.