I'd agree that the Lusitania sinking could lead to a US entry if Germany continued to act badly. So by the summer of 1915 the US could enter the war. This is almost two years ahead of real life, so the effects could be significant. The US had ten dreadnought battleships by then with others soon to be completed. If the US sent some of them to join the Grand Fleet (as they did) the naval balance which was already well in Britain's favor now tips pretty decisively. Germany might not even try to set up an engagement in the North Sea. With the addition in naval strength, perhaps the Allies make a renewed push against the Dardanelles. A victory there would be huge. Even without that, American troops start showing up on the Western Front in significant numbers by mid 1916 forcing the Germans to send their reserves there. This becomes a major boost for the Russians, perhaps enough to stave off the disaster in 1917. I could see the war ending by late 1917 or early 1918.