<P> The United States declared war on Germany on April 6, 1917, during World War I . The U.S. was an independent power and did not officially join the Allies . It closely cooperated with the Allies 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 Pershing, Commander - in - Chief (C - in - C) of the American Expeditionary Force (AEF), arrived in large numbers on the Western Front in the summer of 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 around 45,000 who died due to the 1918 Spanish Flu outbreak (30,000 before they even reached France). 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. Armed Forces . After a relatively slow start in mobilizing the economy and labor 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 U.S. entry into the war . </P> <P> The American entry into World War I came in April 1917, after more than two and a half years of efforts by President Woodrow Wilson to keep the United States out of the war . Apart from an Anglophile element urging early support for the British, American public opinion reflected that of the president: the sentiment for neutrality was particularly strong among Irish Americans, German Americans and Scandinavian Americans, as well as among church leaders and among women in general . On the other hand, even before World War I had broken out, American opinion had been more negative toward Germany than towards any other country in Europe . Over time, especially after reports of atrocities in Belgium in 1914 and following the sinking of the passenger liner RMS Lusitania in 1915, the American people increasingly came to see Germany as the aggressor in Europe . </P> <P> As U.S. President, it was Wilson who made the key policy decisions over foreign affairs: while the country was at peace, the domestic economy ran on a laissez - faire basis, with American banks making huge loans to Britain and France--funds that were in large part used to buy munitions, raw materials and food from across the Atlantic . Until 1917, Wilson made minimal preparations for a land war and kept the United States Army on a small peacetime footing, despite increasing demands for enhanced preparedness . He did however expand the United States Navy . </P> <P> In 1917, with Russia experiencing political upheaval following widespread disillusionment there over the war, and with Britain and France low on credit, Germany appeared to have the upper hand in Europe, while the Ottoman Empire clung to its possessions in the Middle East . In the same year, Germany decided to resume unrestricted submarine warfare against any vessel approaching British waters; this attempt to starve Britain into surrender was balanced against the knowledge that it would almost certainly bring the United States into the war . Germany also made a secret offer to help Mexico regain territories lost in the Mexican--American War in an encoded telegram known as the Zimmermann Telegram, which was intercepted by British Intelligence . Publication of that communique outraged Americans just as German U-boats started sinking American merchant ships in the North Atlantic . Wilson then asked Congress for "a war to end all wars" that would "make the world safe for democracy", and Congress voted to declare war on Germany on April 6, 1917 . On December 7, 1917, the U.S. declared war on Austria - Hungary . U.S. troops began arriving on the Western Front in large numbers in 1918 . </P>

Though the u.s. remained neutral early in the war how did it support the allies