The Americans are obviously outraged by Britain arbitrarily destroying foreign navies that get too big. That won't be enough to get them in on the fighting, but will be to chill relations with London considerably. Then say some clash with the German East Asia squadron leads to some collateral damage in South America, and then that US merchant shipping starts getting interdicted, and later even outright destroyed, by the Royal Navy. America gets fed up and declares war on the Entente, with a weaker Germany a sympathetic ally for the general public. The US tries to invade Canada. By this point, chilled Anglo-American relations has the British reinforcing their border defences, but not dramatically. The place is not exactly an impenetrable fortress, but the fighting is still somewhat tough and the Americans don't rapidly seize the place as they hope. Meanwhile, without having to worry about the Kaiserliches Marine the Royal Navy can concentrate on fighting the American Navy, taking some losses but ultimately coming out on top. Meanwhile, the war in Europe gets too tough for the Germans and they sign an armistice, ceding Alsace-Lorraine and possibly some East Prussian border provinces to the Russians. A-H possibly loses East Galicia and, if Italy joins the Entente, South Tyrol and Dalmatia as well, depending on how the fighting went. Now the Americans are left on their own, but they still hold lots of Canada. The British move to retake the Dominion and end up narrowly beating back the US in a Second War of 1812. They don't penetrate far (or maybe at all) into America proper before a peace is made, essentially restoring the status quo except for some border reductions, war reparations and possibly some reductions on American fleet tonnage.