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An Ottoman Empire without carrying out a genocide will just mean that the existing Armenian Liberation Movement(s) will go largely unchecked or unterrorized, so will continue to go about giving the ailing Turks infinite shit. And if they're going to be left alone then really, like the Irish at this time, the Turkish enemies will no doubt gladly supply Armenian liberation movements the same way the IRA got the German Empire to supply them (although, the events of the 1916 Easter Rising and early 20th century 'Ra is one comedic event after another, culminating in the first shipment of German arms being thrown into the ocean and the man responsible for meeting with the Reich to obtain them being put to death because he was charged of being gay).
As unfortunate as it is, the Ottomans may need to kill a lot of civilians or cede Armenian territory in a civil war possibly backed by whoever they're fighting; and since by the First World War the ground work for the two major axis are already all drawn up there's not a whole lot of wiggle room. The reason X came into the war on Y side wasn't because of an immediate Z event of bit of diplomacy, but Z was something from more a few decades back.
In the instance of America joining the Allies it was probably more to do with the close relationships fostered between the States and Brittania the previous decades and in particular a seemingly minor diplomatic chapter involving the isolation of Germany over their support for Moroccan Independence.