Some six weeks after the United States formally entered the First World War, the U.S Congress passes the Selective Service Act on May 18, 1917, giving the U.S. president the power to draft soldiers.

When he went before Congress on April 2, 1917, to deliver his war message, President Woodrow Wilson had pledged all of his nation’s considerable material resources to help the Allies—France, Britain, Russia and Italy—defeat the Central Powers. What the Allies desperately needed, however, were fresh troops to relieve their exhausted men on the battlefields of the Western Front, and these the U.S. was not immediately able to provide. Despite Wilson’s effort to improve military preparedness over the course of 1916, at the time of Congress’s war declaration the U.S. had only a small army of volunteers—some 100,000 men—that was in no way trained or equipped for the kind of fighting that was going on in Europe.

To remedy this situation, Wilson pushed the government to adopt military conscription, which he argued was the most democratic form of enlistment. To that end, Congress passed the Selective Service Act, which Wilson signed into law on May 18, 1917. The act required all men in the U.S. between the ages of 21 and 30 to register for military service. Within a few months, some 10 million men across the country had registered in response to the military draft.

The first troops of the American Expeditionary Force (AEF), under commander in chief General John J. Pershing, began arriving on the European continent in June 1917. The majority of the new conscripts still needed to be mobilized, transported and trained however, and the AEF did not begin to play a substantial role in the fighting in France until nearly a year later, during the late spring and summer of 1918. By that time, Russia had withdrawn from the conflict due to internal revolution, and the Germans had launched an aggressive new offensive on the Western Front. In the interim, the U.S. gave its allies much-needed help in the form of economic assistance: extending vast amounts of credit to Britain, France and Italy; raising income taxes to generate more revenue for the war effort; and selling so-called liberty bonds to its citizens to finance purchases of products and raw materials by Allied governments in the United States.

By the end of World War I in November 1918, some 24 million men had registered under the Selective Service Act. Of the almost 4.8 million Americans who eventually served in the war, some 2.8 million had been drafted.

READ MORE: Life in the Trenches of World War I