Complex sentence: Four divisions of the U.S.10th Army (the 7th, 27th, 77th, and 96th) and two Marine Divisions (the 1st and 6th) fought on the island, supported by naval, amphibious, and tactical air forces.
Split into simpler sentences: Four divisions of the U.S.10th Army (the 7th, 27th, 77th, and 96th) and two Marine Divisions (the 1st and 6th) fought on the island. Their invasion was supported by naval, amphibious, and tactical air forces.