From about 1400, despite its incorporation into the Japanese political order (this incorporation was however limited, to the point that Japanese authorities, regional and national, were unable throughout most of Japanese history to control and limit pirate activity originating in this area) before the Goryeo, Tsushima were located on the front lines that defended Japanese territory for much of its history. Historically, a large part of Tsushima's economy was sustained by trade with Korea; it was used as a "frontier territory" and a diplomatic meeting place between Korea and Japan, but was considered historically by Koreans as a vassal or dependent state of Korea, and despite a variety of changes in terminology over the ages designed to indicate its status as being in the Japanese sphere of influence, it was considered by many Koreans to be Korean land under foreign occupation.