the american geopolitical goal in the middle east is to maintain a careful balance of power. the americans want each of the powers fighting against each other, without any of them taking control. they'll shift the balances to ensure that this power stays in place. the primary architect of this system was kissinger, who was drawing mostly from bismarck. it's that kind of realpolitik.
so, if the americans wake up and decide that the saudis are getting out of hand, they may covertly support the iranians to balance it out. if they decide the turks are gaining too much influence, they may produce instability in the kurdish regions. if israel is demonstrating too much strength, they may force a concession out of them. of course, these are all american allies, and everything that happens is in the context of each country's dependence on america - which is what america wishes to maintain. the description of "great satan" is actually quite apt, in the way that america sows chaos amongst it's own allies.
isis may have come from al qaeda in iraq, but what's happening right now is a corollary of the arab spring, which was used as an opportunity to reshape the region into an empire centered around riyadh. the thing that defines the new leadership in the middle east is that it is all very closely aligned with the saudis. but, there has been some disagreement about the way it's been done. if you want to look at an existing caliphate, the reality is that iraq and syria are the last remaining arab states that are not under the control of the saudis.
assad, in particular, made an error that cannot be forgiven - he threatened to call elections. this is unacceptable to a saudi regime that sees democracy as it's greatest threat. that democracy may be less real than we'd like, but it's not the point. it's the idea that cannot be tolerated. the israelis seem to share a lot of these real hard line attitudes, for their own reasons.
on the other hand, you have a coalition including the qataris and turks that considers the saudis to be too extreme and wishes to temper their influence. this was the fight between "moderate" and "islamist" forces in syria - and remember they were legitimately killing each other. it was a proxy war between turkey and saudi arabia, and the turks lost.
now, we have this situation: the saudis are getting out of control, so the americans need to step in on behalf of the turks to clean up the border region. but, not to the point that isis is demolished. that would tip the balance of power...
is it an excuse into syria? it seems like an opportunity, but i don't believe the united states army requires an excuse to do anything at all at this point. it's really about pushing the saudis away from turkey, to prevent them from fighting each other and maintain that balance of power. all the other layers (russia v us, saudi v iran, etc) are there, but i think that this is the key one.