The Lusitania was NOT an American ship, but it did have quite a few American passengers on board and THAT was the excuse that was used for Wilson to get us into WW I; not about American ships, of any kind, being in harms way. Until then, America was a NEUTRAL country and the Germans even let our Government know that Americans shouldn't travel on that ship, prior to its last voyage.
This war had no good guys and no bad guys. I am still amazed that the Germans could be involved in 3 of the largest battles in military history at the same time (Somme, Brusilov, Verdun) and still survive.