1917 – American entry into World War I: President Wilson asks the U.S. Congress for a declaration of war on Germany. The United States entered into World War I on 6 April 1917, more than two and a half years after the war began in Austria-Hungary. 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 had generally reflected a desire to stay out of the war. Over time, especially after reports of German atrocities in Belgium in 1914 and after the sinking of the RMS
Lusitania in a torpedo attack by a submarine of the Imperial German Navy off the southern coast of Ireland in May 1915, Americans increasingly came to see Imperial Germany as the aggressor in Europe. When the country was at peace, American banks made huge loans to the Entente powers (Allies), which were used mainly to buy munitions, raw materials, and food from across the Atlantic in North America from the United States and Canada. Although president Woodrow Wilson made minimal preparations for a land war before 1917, he did authorize a shipbuilding program for the United States Navy. Wilson was narrowly re-elected in 1916 on an anti-war platform. By 1917, with Belgium and northern France occupied by German troops, the Russian Empire experiencing turmoil and upheaval in the February Revolution overthrowing the tsar on the Eastern Front, and with the remaining Entente nations low on credit, the German empire appeared to have the upper hand in Europe. However, a British economic embargo and naval blockade were causing severe shortages of fuel and food in Germany. Berlin then decided to resume unrestricted submarine warfare. The aim was to break the trans-Atlantic supply chain to Britain from other nations to the West, although the German high command realized that sinking American-flagged ships would almost certainly bring the United States into the war. Imperial Germany also made a secret offer to help Mexico regain territories of the Mexican Cession of 1849, lost seven decades before in the Mexican–American War of 1846–1848, (now incorporated in the Southwestern United States) in an encoded diplomatic secret telegram known as the Zimmermann Telegram intercepted by British intelligence. Publication in the media of that communique outraged Americans just as German submarines started sinking American merchant ships in the North Atlantic in their U-boat campaign. President 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. US troops began to arrive in Europe later that year, and served in major combat operations on the Western Front under the command of general John J. Pershing.
