Originally, the U.S. proclaimed its neutrality in the conflict; although it was probably inevitable that the U.S. would eventually become involved.
U.S. opinion settled firmly on the Allied side following the sinking of RMS Lusitania in 1915. Although Germany promised to no longer sink ships with its submarines without warning, and Woodrow Wilson commented that "there is such a thing as being to proud to fight," it was then only a matter of time. Part of the problem had been that the U.S. was sending supplies and other war materiel to the Allies which managed to avoid the German blockade of the British Isles. When Germany informed the U.S. that it would resume unrestricted submarine warfare (sinking ships without warning,) the U.S. broke diplomatic relations with Germany. Finally, when the British intercepted the infamous Zimmerman telegram, by which Germany sought to induce Mexico to come into the war against the U.S., President Wilson sent a war message to Congress. A formal declaration of war soon followed.