Complex sentence: The U-Boat fired a torpedo and made no provision for any survivors, under Germany's policy of unrestricted submarine warfare but against the Imperial German Navy's own restriction on attacking passenger liners, the "Arabic" pledge.
Split into simpler sentences: The U-Boat fired a torpedo and made no provision for any survivors. This action took place under Germany's policy of unrestricted submarine warfare, but broke the Imperial German Navy's own restriction on attacking passenger liners, the "Arabic" pledge.