Complex sentence: A third journey, planned for January 1917, was however aborted when the U.S. entered the war against Germany--partly because of U.S. anger over the actions of German submarines sinking shipping bound for Great Britain, sometimes just outside of American territorial waters.
Split into simpler sentences: A third journey, planned for January 1917, was aborted after the U.S. entered the war against Germany. The declaration of war had been partly because of U.S. anger over the actions of German submarines sinking shipping bound for Great Britain, sometimes just outside of American territorial waters.