By the winter of 1916/17, World War I had reached a deadlock. While the Allies commanded greater resources and fielded more soldiers than the Central Powers, German armies had penetrated deep into Russia and France, and tenaciously held on to their conquered empire. Hoping to break the stalemate on the western front, the exhausted Allies sought to bring the neutral United States into the conflict.A golden opportunity to force American intervention seemed at hand when British naval intelligence intercepted a secret telegram detailing a German alliance offer to Mexico....