Katō Kiyomasa was one of the most powerful and well-known lords of the Sengoku Era. He commanded most of Japan's major clans during the invasion of Korea (1592–1598). In a handbook he addressed to "all samurai, regardless of rank" he told his followers that a warrior's only duty in life was to "...grasp the long and the short swords and to die". He also ordered his followers to put forth great effort in studying the military classics, especially those related to loyalty and filial piety. He is best known for his quote: "If a man does not investigate into the matter of Bushido daily, it will be difficult for him to die a brave and manly death. Thus it is essential to engrave this business of the warrior into one's mind well."