American entry into World War I
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 and an anti-Tsarist element sympathizing with Germany's war against Russia, 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 women in general. On the other hand, even before World War I had broken out, American opinion had been overall more negative toward Germany than toward any other country in Europe. Over time, especially after reports of German atrocities in Belgium in 1914 and following the sinking of the passenger liner RMS Lusitania in 1915, American citizens increasingly came to see Germany as the aggressor in Europe.
While the country was at peace, American banks made huge loans to the Entente powers, which were used mainly to buy munitions, raw materials, and food from across the Atlantic. Wilson made minimal preparations for a land war but he did authorize a major ship-building program for the United States Navy. The president was narrowly re-elected in 1916 on an anti-war ticket.
In 1917, with Russia experiencing political upheaval, and with the remaining Entente nations low on credit, Germany appeared to have the upper hand in Europe, while the Ottoman Empire, Germany's ally, held on to its territory in modern-day Iraq, Syria and Palestine. However, an Entente economic embargo and naval blockade was by now causing shortages of fuel and food in Germany, at which point Germany decided to resume unrestricted submarine warfare. The aim was to break the transatlantic supply chain to Britain from other nations, although the German high command realized that sinking American-flagged ships 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 submarines 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. U.S. troops began major combat operations on the Western Front under General John J. Pershing in the summer of 1918.