During the Seven-Year Korean War, he is noted for the defeat of a much larger "Ming army" at the siege of Ulsan castle.

However, when Hidemoto ordered an assault, Hiroie refused to comply and used the vanguards under his command to block off the attack routes.

Ieyasu believed the documents showed that Terumoto was involved more deeply in the western army than Hiroie had presented, and thus voided their secret pact.

Many considered him a traitor, especially since it was felt that his actions during the battle was instrumental in the Tokugawa victory that led to the disastrous punishment.

Instead, the Kikkawa family continued to be senior retainers of the Mōri clan until the end of the Bakufu, at which point the Iwakuni domain officially became a han.

Hiroie worked for prosperity of his domain; he is remembered for having laid down a set of laws with 188 clauses (the Kikkawa-shi hatto).