During the year 1582, Harunobu lost Shimabara Castle and was reduced to holding a thin strip of the peninsula. Harunobu called for the help of the Shimazu clan against the forces of Ryūzōji Takanobu. The Shimazu sent Shimazu Iehisa to Shimabara. During the year 1584, the combined forces of the Arima and the Shimazu, with over 3,000 troops, defeated that of the Ryūzōji. The battle that they fought was known as the Battle of Okitanawate. During that battle, Ryūzōji Takanobu was killed. Afterwards, Shimazu Yoshihisa suggested that the Arima renounce Christianity, but this was only refused by Harunobu.