The sinking of the Lusitania was a tragic event which ultimeately brought the USA into the war in 1917.
At that point the Germans were focusing theur naval resources on unrestricted warfare, by which, any vessel deemed to be transporting war materials to Britain would be sunk.
Although not known at the time, the Lusitania was carrying munitions under the orders of Winston Churchill who was First Lord of the Admiralty in 1915.
The backlash against Germans, Germany and its U-Boats was as expected. In London, German shops and families were attacked and the British Government used the sinking to fuel their anti-German propaganda.