<P> Once Germany had decided on unrestricted submarine warfare in January 1917, and knowing it would be attacking all American ships in the North Atlantic, it tried to line up new allies, especially Mexico . Arthur Zimmermann, the German foreign minister, sent the Zimmermann Telegram to Mexico on January 16, 1917 . Zimmerman invited Mexico (knowing their resentment towards America since the 1848 Mexican Cession) to join in a war against the United States . Germany promised to pay for Mexico's costs and to help it recover the territory annexed by the U.S. in 1848 . These territories included the present day states of California, Nevada, Utah, most of Arizona, about half of New Mexico and a quarter of Colorado . British intelligence intercepted and decoded the telegram and passed it to the Wilson administration . The White House would release it to the press on March 1st . Anger grew further as the Germans began sinking American ships, even as isolationists in the Senate launched a filibuster to block legislation for arming American merchant ships to defend themselves . </P> <P> In early 1917 Berlin forced the issue . Its declared decision on 31 January 1917 to target neutral shipping in a designated war - zone became the immediate cause of the entry of the United States into the war . Five American merchant ships went down in March . Outraged public opinion now overwhelmingly supported Wilson when he asked Congress for a declaration of war on April 2, 1917 . </P> <P> Historians such as Ernest R. May have approached the process of American entry into the war as a study in how public opinion changed radically in three years' time . In 1914 most Americans called for neutrality, seeing the war a dreadful mistake and were determined to stay out . By 1917 the same public felt just as strongly that going to war was both necessary and wise . Military leaders had little to say during this debate, and military considerations were seldom raised . The decisive questions dealt with morality and visions of the future . The prevailing attitude was that America possessed a superior moral position as the only great nation devoted to the principles of freedom and democracy . By staying aloof from the squabbles of reactionary empires, it could preserve those ideals--sooner or later the rest of the world would come to appreciate and adopt them . In 1917 this very long - run program faced the severe danger that in the short run powerful forces adverse to democracy and freedom would triumph . Strong support for moralism came from religious leaders, women (led by Jane Addams), and from public figures like long - time Democratic leader William Jennings Bryan, the Secretary of State from 1913 to 1916 . The most important moralist of all was President Woodrow Wilson--the man who dominated decision making so totally that the war has been labelled, from an American perspective, "Wilson's War". </P> <P> In 1917 Wilson won the support of most of the moralists by proclaiming "a war to make the world safe for democracy ." If they truly believed in their ideals, he explained, now was the time to fight . The question then became whether Americans would fight for what they deeply believed in, and the answer turned out to be a resounding "Yes". </P>

Which was a major cause for the entry of the us into world war i