Complex sentence: The first day of the Dutch Harbor air raid inflicted very little damage on the U.S. forces in the area, and was entirely ineffective as a diversion since the Americans knew that the main Japanese offensive was to be at Midway island.
Split into simpler sentences: The first day of the Dutch Harbor air raid inflicted very little damage on the U.S. forces in the area. The Japanese attack was entirely ineffective as a diversion since the Americans knew that the main Japanese offensive was to be at Midway island.