America’s policy of insisting on neutral rights while also trying to broker a peace resulted in tensions with both Berlin and London. When a German U-boat sank the Lusitania in 1915, a large passenger liner with 128 Americans aboard, Wilson vowed "America was too proud to fight," and demanded an end to attacks on passenger ships. Germany complied. Wilson tried to mediate a compromise settlement; yet no compromise was discovered. Wilson also repeatedly warned that America would not tolerate unrestricted submarine warfare because it violated America's rights. Wilson was under great pressure from former president Teddy Roosevelt, who denounced German "piracy" and Wilson's cowardice. In January 1917 the Germans announced they would resume unrestricted submarine warfare. Berlin's proposal to Mexico to join the war as Germany's ally against the U.S. was exposed in February, angering American opinion. (see Zimmermann Telegram). After German submarines attacked several American merchant ships, sinking three, Wilson requested that Congress declare war on Germany, which it did on April 6, 1917.[11] The U.S. House of Representatives approved the war resolution 373-50, the U.S. Senate 82-6, with opposition coming especially from German American districts such as Wisconsin. The U.S. declared war on Austria-Hungary in December 1917.
The United States was never formally a member of the Allies but an “Associated Power”. Significant numbers of fresh American troops arrived in Europe in the summer of 1918, and they started arriving at 10,000 per day. Germany miscalculated that it would be many more months before large numbers of American troops could be sent to Europe, and that, in any event, the U-boat offensive would prevent their arrival. In fact, not a single American infantryman lost his life due to German U-boat activity.
The United States Navy sent a battleship group to Scapa Flow to join with the British Grand Fleet, several destroyers to Queenstown, Ireland, and several submarines to the Azores and to Bantry Bay, Ireland, to help guard convoys. Several regiments of U.S. Marines were also dispatched to France. However, it would be some time before the United States would be able to contribute significant personnel to the Western and Italian fronts.
