By the spring of 1915, it was clear that the war would last for a long time and that it would be taking an economic toll on all the nations involved, and there would likely be political consequences as well. In Britain, the debate centered on whether the government was doing enough to support the French, and in particular, whether British soldiers were being supplied with enough artillery shells to get the job done.
In early 1915, with the Western Front in a stalemate, Winston Churchill becomes the leading voice behind a plan to do an end run around the Germans and knock the Turks out of the war.
In the spring of 1915, just before the sinking of Lusitania and in international women's conference aimed at ending the war through private diplomacy, Germany uses poison gas on the Western Front.
The sinking of Lusitania and the deaths of 128 Americans was a shock. While there was little support in the US for war against Germany, there was a strong feeling that *some* kind of response was necessary. It was up to Woodrow Wilson to figure out what that would be.