However, Sadashige SO, who had tried hard to restrict Wako activity, fell gravely ill in September 1417 before eventually dying of this illness in April 1418, and when Sadamori SO succeeded Sadashige as ruler of Tsushima, the So clan's ability to police the seas around Tsushima was hampered and weakened by Sadamori's youth and inexperience; this meant that the Wako, who had been held in check by Sadashige, erupted into fresh activity near Tsushima, inflicting heavy damage on Japan and Korea.
しかし、倭寇の取り締まりを積極的に行っていた宗貞茂が1417年9月、病に倒れ翌1418年4月に病没し、宗貞盛が跡目を継ぐと、若年の当主のため対馬の周辺海域の警察力が弱体化し、活動を抑制されていた倭寇は再び活発化し、日本や朝鮮で多大な被害が出ていた。