All through the spring and summer of 1916, Wilson and House tried to persuade Sir Edward Grey to put the House-Grey Memorandum into effect. Grey adamantly refused, and Wilson abandoned all hope of peace through Anglo-American cooperation. This development and others embittered Anglo-American relations and caused Wilson to believe that the British were prolonging the war for conquest and revenge. Ever since the outbreak of the war, Wilson had hoped to bring it to an end. By late 1916, the conflict seemed about to destroy the very fabric of European society; moreover, Wilson knew that both sides would intensify their efforts to end the bloody struggle, and intensification of the war at sea would probably force the United States into the conflict.
To bring peace to the world and to avert the possibility of American participation, Wilson appealed to the belligerents on 18 December 1916 to disclose the terms upon which they would agree to end the fighting. The Germans refused to divulge their terms. The Allies—emboldened by Lansing's assurances, made secretly to the French and British ambassadors, that Wilson was pro-Ally—announced terms that could be achieved only by complete defeat of the Central Powers. Undaunted, Wilson opened secret negotiations with the German government. He was, he told Berlin, prepared to be an independent and impartial mediator, and he could force the Allies to the peace table because they were now totally dependent upon American credit and supplies. While he waited for a reply from Berlin, Wilson went before the Senate on 22 January 1917 to describe the kind of a peace settlement that the United States was prepared to work for and support. It had to be a "peace without victory," he said, one without indemnities and annexations. Above all, it had to be based upon a league of nations to preserve peace.
The Germans were as much determined upon total victory as the Allies. They abhorred the idea of Wilson's mediation and believed that their now large fleet of long-range submarines could bring the British to their knees long before the United States could send a single soldier to France. Hence, they rejected Wilson's hand of friendship, accepted the prospect of war with the United States, and announced on 31 January 1917 that they would begin, the following day, a ruthless submarine campaign against all merchant shipping in European waters.
Wilson was stunned, and he broke diplomatic relations with the German Empire on 3 February. Even so, he said that he hoped that no German aggressions against American ships would force the United States to take sterner measures of protection. American ships refused to enter the war zone, and the nation waited expectantly. Then, in late February, the British disclosed a telegram from the German foreign secretary, Arthur Zimmermann, to the German minister in Mexico. The "Zimmermann telegram" instructed the minister, in the event that the United States entered the war against Germany, to offer to the Mexican government an alliance by which Mexico would go to war against the United States and would receive in return "the lost territory in Texas, New Mexico, and Arizona."
Events thereafter led inexorably to war between the United States and Germany. On 9 March, Wilson announced that the navy would place guns and gun crews on American merchantmen. It soon became obvious that armed neutrality would not suffice to protect American rights, and Wilson, after much agonizing, on 2 April asked Congress to recognize that a state of war already existed between the United States and Germany on account of German aggressions. In a moving peroration, Wilson declared that the world had to be made "safe for democracy" and freed from the threat of German militarism. Congress complied after a brief debate, and Wilson signed the war resolution on 6 April 1917.