Complex sentence: Meanwhile, preparations to invade the Solomon Islands were proceeding to disrupt the Japanese offensive to establish a defensive perimeter around the edge of their "Greater East Asia Co-Prosperity Sphere".
Split into simpler sentences: Meanwhile, preparations to invade the Solomon Islands were proceeding apace. Up to that point, the Japanese had been on the offensive, establishing their defensive perimeter around the edge of their "Greater East Asia Co-Prosperity Sphere".