In the early 20th century, Americans tended to be isolationists when it came to foreign policy. For the most part, WWI looked like a European conflict into which America need not enter, and president Woodrow Wilson pledged to keep the country out of the conflict. However, after Germany continued to attack unarmed merchant and passenger ships the U.S. severed diplomatic ties with it.
In January 1917, British cryptographers deciphered a communication between German Foreign Minister Arthur Zimmermann to the German Minister to Mexico. In it, Zimmermann offered United States territory to Mexico in return for joining the German cause. Britain shared the telegram with Wilson, and U.S. papers published it in March. On April 6, 1917, Congress formally declared war on Germany (and its allies).
Public opinion about the war underwent a great change from its previous neutrality. Everyone was expected to help win the war. Women worked on the home front by serving meals that spared essential foodstuffs like grain, ran their households as frugally as possible, and filled in for men who left home for the military. Men found it very difficult to resist the call to arms. And, just as Native Americans joined campaigns to buy Liberty Bonds (see last post), they supported the war effort by entering the military.