When an envoy from Korai (Korea) came to Tsushima with a message of Mongolia to Japan (1267), he took the Korean envoy to Dazaifu, the seat of the Shogunate military headquarters in Kyushu, at the latter's request. When Mongolia afterward threatened to invade Japan, Sukekuni bolstered the defenses of Tsushima at the order of the Shogunate.
A joint invasion force of Mongolia and Кorаі stormed Sasuura Beach of Tsushima (1274). Sukekuni reported the invasion to Dazaifu and endeavored in vain to defend the island against the invaders together with his small force of more than 80. He, his sons Umajiro and Yajiro killed themselves in defeat.