Complex sentence: He also intended to occupy Midway Atoll as part of an overall plan to extend Japan's defensive perimeter in response to the Doolittle Raid; afterward, it would be turned into a major airbase, giving Japan control of the central Pacific.
Split into simpler sentences: He also intended to occupy Midway as part of an overall plan to extend Japan's defensive perimeter in response to the Doolittle Raid. It would then be turned into a major airbase, giving Japan control of the central Pacific.