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.

No we should just pass