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.
