This game is based on the first Mongol invasion of Japan during 1274 (late 13th century). This invasion is by Kublai Khan, who is the grandson of Genghis Khan. The “Tsushima” in the Ghost of Tsushima game refers to the Tsushima Island in Japan, which is real.
Here is the satellite photo of the island taken by NASA:
This place is around 50 km from Busan, South Korea, which is the closest to the Japanese territory.