What event in 1915 caused many Americans to become angry at Germany?

When World War I began in 1914, the United States - - which had never interfered in a European war - - was officially neutral. The land war quickly devolved into a stalemate as the Central Powers - - consisting of Germany, Austria-Hungary, Bulgaria and the Ottoman Empire - - and the Allies - - consisting of the British Empire, France, Belgium, Italy and Russia - - dug extensive series of trenches. At sea, the British Empire's surface fleet was dominant, leading the German Empire to declare unrestricted submarine warfare around the British Isles.