What does the red mean?

[v0031]

Last and biggest of the Pacific island battles of World War II, the Okinawa campaign (April 1—June 22, 1945) involved the 287,000 troops of the U.S. Tenth Army against 130,000 soldiers of the Japanese Thirty-second Army. At stake were air bases vital to the projected invasion of Japan. By the end of the 82-day campaign, Japan had lost more than 77,000 soldiers and the Allies had suffered more than 65,000 casualties—including 14,000 dead.

[True Federalist (진정한 연방 주의자)]

While bombers could make it to Japan from the Marianas which the US had already taken, fighters needed a closer location. American fighters based in Okinawa could provide air coverage for the invasion of Kyushu which was the next target.  (Even from Okinawa, the fighters couldn't have reached the main island of Honshu.) While a few fighters might have made it to Japan from Iwo Jima, it is too small to base many planes from and was taken mainly as a place damaged bombers could land at if they couldn't make it back to the Marianas.