On April 2, 1917, President Woodrow Wilson addressed a joint session of Congress to ask for a declaration of war against Germany. Congress granted the request on April 6 and launched the United States into what was known as The Great War and later, World War I.
President Wilson wrestled with his decision to declare war. His campaign slogan the previous year was, “he kept us out of war,” and is credited with his narrow win. In his address to Congress, Wilson uttered the now-famous phrase, “The world must be made safe for democracy." He added that he had "a distressing and oppressive duty" and that it was "a fearful thing to lead this great peaceful people into war, into the most terrible and disastrous of all wars, civilization itself seeming to be in the balance."
The Zimmerman Telegram is credited as the final provocation. The British intercepted a coded message from German Foreign Secretary Arthur Zimmerman to the German Ambassador to Mexico. In it, Germany offered Mexico territory it had lost to the U.S. in New Mexico, Texas, and Arizona as a reward for Mexico’s support in the war.
Once they received news of the telegram, newspapers across the nation reacted with banner headlines. On March 1, 1917, the Chicago Tribune printed perhaps the largest and most succinct: "U.S. Bares War Plot." The San Francisco Chronicle called the telegram an "amazing letter … which discloses the German plot to embroil the United States with Mexico and Japan."
More than 600,000 documents relating to the war, including State Department Records, Gorrell’s History-AEF Air Service, and WWI Military Cablegrams can be found in the World War I Collection on Fold3.