Complex sentence: The landings were meant to deny their use to the Japanese as bases, especially the nearly completed airfield abuilding on Guadalcanal, for from the eastern Solomons, Japanese air and sea forces threatened the supply routes between the U.S. and Australia.
Split into simpler sentences: The landings were meant to deny their use to the Japanese as bases. From the eastern Solomons, Japanese forces threatened the supply routes between the U.S. and Australia.