Complex sentence: Airfields were being constructed as the battle was still being fought, but the delay in the start of the Palaus operation meant that the airfields were not ready in time for the start of the Philippines operations in October 1944.
Split into simpler sentences: Airfields were being constructed as the battle was still being fought. But the delay in the start of the Palaus operation meant that the airfields were not ready in time for the start of the Philippines operations in October 1944.