In 1638, the 75-year-old Katsunari led troops to suppress the Shimabara Rebellion. Despite needing support from his sons and grandsons to reach the battlefield, his fighting spirit was undiminished. He overruled the siege strategy, ordering an immediate assault that broke the stalemate. After the war, he furiously protested the shogunate's dismissal of the rebels as 'mere peasants,' insisting they were heroes who fought for their beliefs, and retired in disgust.