On 6 April 1917, American president Woodrow Wilson declared war on Germany. Until that day, the United States had remained neutral. The declaration of war was a response to the submarine war that Germany had been waging on its enemies since January 1917.
Germany wanted to sink all ships sailing toward the United Kingdom, including passenger ships and neutral American ships, as they could be carrying aid - from food to soldiers - for Germany's opponents. Germany knew that the United States would respond, but they believed that the United States would take so long to prepare, that the British would have quit their efforts by then. But they were wrong.
Another reason for the American declaration of war was the interception of the so-called "Zimmermann telegram”, in which Germany promised Mexico financial support and American territory if it would attack the United States. Mexico refused the request. The telegram had been sent in code, but it was deciphered by the English secret service. The Americans were outraged.
In 1917, the United States sent more than a million troops to France. It took another year before they were fighting alongside the Allies in substantial numbers.