Complex sentence: The first ship sunk was the neutral Dutch steamer, which provoked outrage in the Netherlands and helped turned Dutch public opinion against Germany, and was the impetus behind an renewed effort to avoid attacking neutral ships by U-boats.
Split into simpler sentences: The first ship sunk, the neutral Dutch steamer, provoked outrage in the Netherlands and helped turned Dutch public opinion against Germany. That sinking was also the impetus behind a renewed effort to avoid attacking neutral ships by U-boats.