President Woodrow Wilson declared the United States would remain neutral when World War I broke out in Europe in 1914, and many Americans backed this noninterventionist stance. But after the German U-boat sank the British ocean liner Lusitania in 1915, killing over 2,000 people—including 128 Americans—public opinion about neutrality began to shift.
Wilson requested a declaration of war against Germany before a special joint session of Congress on April 2, 1917, saying that “the globe must be made safe for democracy.” On April 4, the Senate voted 82 to 6 to declare war on Germany. The House of Representatives adopted a war resolution against Germany by a vote of 373 to 50 on April 6, two days later.
Founded in 1917, the U.S. Army only had 133,000 people in it. By the end of World War I, some 2.8 million men had been enlisted in the American military as a result of the Selective Service Act, which Congress approved in May and restored the draft for the first time since the American Civil War. During the conflict, an additional 2 million Americans volunteered to join the military.
The first American infantry forces landed in Europe in June 1917, and the first American soldiers engaged in battle in France in October. In December of that year, America declared war on Austria-Hungary (America never was formally at war with the Ottoman Empire or Bulgaria).
More than 2 million American soldiers had served at the Western Front in Europe by the time the war ended in November 1918, with a victory for the Allies, and more than 50,000 of them had died.
REF: History.com
PIC: History.com, CNN, Encyclopedia Britannica, MPR News, World War | Centennial Commission