So I was reading about the Mongol attempts at conquering Japan. In both cases, their invading forces left from Korea or China, and landed in the southernmost portion of the island chain. The weather played a considerable role in deterring their forces, and ultimately they were defeated.
However, I also read that the Mongol Empire had already subjugated the Ainu in Sakhalin by this time. The crossing between Sakhalin and the north of Japan appears to be a much shorter distance than that of Japan and Korea. So why had they chosen Korea as their staging area instead?
The crossing between Japan and Korea is at its shortest actually fairly short, its only a few hours of sailing from Korea to Tsushima and on to Kyushu. Invading Japan through Sakhalin would have been impractical as hell - sakhalin was extremely remote, and the Mongols had severe problems subjugating the local Ainu, so much that Sakhalin wasn't fully conquered before 1308, 30 years after the invasions of Japan, and also, the Japanese civilization at that point was centered in southern and central Japan.
So the Mongols would have to move hteir enitre army to the Amur river through Manchuria, cross to Sakhalin, then cross again to Hokkaido (and mind you nobody at the time knew what this part of the world looked like), then cross form Hokkaido to Honshu, and march down to Kamakura and Kyoto - this was highly impractical compared to just sailing across from conquered Korea, and they had to built the navy as well - remember than half of the fleet of the second invasion actually sailed form Yangzhou in southern China, not Korea. Hope that helped!