The sinking of RMS Lusitania, contributed to America's entry to the War. The shocking story of the sinking and huge loss of life is recorded by Wikipedia HERE.
The Imperial War Museum says one of the most famous commemorative medals in the Art Department's collection is Karl Goetz's 'Lusitania Medallion'. It was privately issued in Germany to mark the circumstances of the sinking of the Cunard liner RSM Lusitania in May 1915. British copies of the piece were subsequently widely distributed as part of a concerted anti-German propaganda campaign. I have a couple of boxed British copies along with the accompanying facsimile newspaper article. A detailed article by the Museum can be found HERE. It makes fascinating reading.