The United States of America declared war on the German Empire on April 6, 1917. The U.S. was an independent power and did not officially join the Allies. It closely cooperated with them militarily but acted alone in diplomacy. The U.S. made its major contributions in terms of supplies, raw material and money, starting in 1917. American soldiers under General John J. Pershing, commander of the American Expeditionary Force (AEF), arrived in large numbers on the Western Front in the summer of 1918. They played a major role until victory was achieved on November 11, 1918. Before entering the war, the U.S had remained neutral, though it had been an important supplier to Great Britain and the other Allied powers. During the war, the U.S mobilized over 4 million military personnel and suffered 110,000 deaths, including 43,000 due to the influenza pandemic. The war saw a dramatic expansion of the United States government in an effort to harness the war effort and a significant increase in the size of the U.S. military. After a slow start in mobilising the economy and labour force, by spring 1918 the nation was poised to play a role in the conflict. Under the leadership of President Woodrow Wilson, the war represented the climax of the Progressive Era as it sought to bring reform and democracy to the world, although there was substantial public opposition to United States entry into the war.
Although the United States declared war on Germany on April 6, 1917, it did not initially declare war on the other Central Powers, a state of affairs that Woodrow Wilson described as an "embarrassing obstacle" in his State of the Union speech. Congress declared war on the Austro-Hungarian Empire on December 17, 1917, but never made declarations of war against the other Central Powers, Bulgaria, the Ottoman Empire or the various Co-belligerents allied with the central powers, thus the United States remained uninvolved in the military campaigns in central, eastern and southern Europe, the Middle East, the Caucasus, North Africa, Sub-Saharan Africa, Asia and the Pacific.
The United States as late as 1917 maintained only a small army, smaller than thirteen of the nations and empires already active in the war. After the passage of the Selective Service Act in 1917, it drafted 2.8 million men into military service. By the summer of 1918 about a million U.S. soldiers had arrived in France, about half of whom eventually saw front-line service; by the Armistice of November 11 approximately 10,000 fresh soldiers were arriving in France daily. In 1917 Congress gave U.S. citizenship to Puerto Ricans when they were drafted to participate in World War I, as part of the Jones Act. In the end Germany miscalculated the United States' influence on the outcome of the conflict, believing it would be many more months before U.S. troops would arrive and overestimating the effectiveness of U-boats in slowing the American buildup.
The United States Navy sent a battleship group to Scapa Flow to join with the British Grand Fleet, destroyers to Queenstown, Ireland and submarines to help guard convoys. Several regiments of Marines were also dispatched to France. The British and French wanted U.S. units used to reinforce their troops already on the battle lines and not to waste scarce shipping on bringing over supplies. The U.S. rejected the first proposition and accepted the second. General John J. Pershing, American Expeditionary Forces (AEF) commander, refused to break up U.S. units to serve as mere reinforcements for British Empire and French units. As an exception, he did allow African-American combat regiments to fight in French divisions. The Harlem Hellfighters fought as part of the French 16th Division, earning a unit Croix de Guerre for their actions at Château-Thierry, Belleau Wood, and Séchault.
Impact of US forces on the war
On the battlefields of France in spring 1918, the war-weary Allied armies enthusiastically welcomed the fresh American troops. They arrived at the rate of 10,000 a day, at a time when the Germans were unable to replace their losses. After British Empire, French and Portuguese forces had defeated and turned back the powerful final German offensive (Spring Offensive of March to July, 1918), the Americans played a role in the Allied final offensive (Hundred Days Offensive of August to November). However, many American commanders used the same flawed tactics which the British, French, Germans and others had abandoned early in the war, and so many American offensives were not particularly effective. Pershing continued to commit troops to these full- frontal attacks, resulting in high casualties against experienced veteran German and Austrian-Hungarian units. Nevertheless, the infusion of new and fresh U.S. troops greatly strengthened the Allies' strategic position and boosted morale. The Allies achieved victory over Germany on November 11, 1918 after German morale had collapsed both at home and on the battlefield.