In early 1917 the United States was finally pushed into the war. On January 31, Germany resumed unrestricted submarine warfare, in an attempt to halt the imports of food and supplies into Britain. Germany knew that this act was likely to bring the United States into the war. However, the Germans concluded that the effects of the submarine campaign would cause the British to capitulate before United States military aid could have an impact on the course of the war.

After the loss of several United States ships to German submarine attacks and the disclosure of the Zimmerman Note (a note to the German ambassador to Mexico informing him of a plan to forge an alliance with Mexico against the United States), the Wilson administration decided on March 30 that there was no alternative to war with Germany. A special session of Congress convened April 2 and Wilson asked that war be declared. The Senate passed the war resolution on April 4, the House on April 6. Wilson promptly signed the resolution and the United States was in the war.

Preparations were made for raising and equipping a United States army as strong as the armies of the European powers. Wilson selected General John J. Pershing to head the American Expeditionary Forces (A.E.F.) and on May 18 ordered an army division sent to France. Pershing and his staff arrived in London June 8.

Congress passed the Selective Service (draft) Act on May 18, requiring registration of all men aged 21 through 30. Cantonments (training camps) and airfields were established and by September some 600,000 draftees were in training. Before the end of the year, the United States was prepared to send more troops to France than the supply transports could carry. By October, the first troops were ready to take positions at the front.