Ghost of Tsushima Wiki Answers

Why did the Mongol invade Tsushima Island?

The Mongols invaded Tsushima Island because of its strategic position. It lies between Japan and Korea. Genghis Khan wants to claim the whole of Japan, and it only makes sense that he takes the first island where they can rest and replenish supplies.

In essence, Tsushima is just a resting spot for the Mongol warships, as the first major battles will occur at the Hakata Bay.