Well, as a casual observer, a few things seem logical/likely to me:

1. The Obama administration seems to have wisely determined that the artificial, temporary stability achieved in Iraq under the original plan was neither durable, nor anything we could hope to artificially sustain at reasonable costs. Besides, to what benefit?

2. Concern about the de facto shift in the Shia-Sunni line of competition; moving it from the Iraq-Iran border to the borders of Lebanon, Jordan and Saudi Arabia; it made sense to back the Saudi scheme of pushing that line back up into Iraq.

3. By providing aid to any Sunni rebel group in Syria we essentially made that aid available to every Sunni rebel group in Syria and Iraq. I imagine the leadership in KSA and DC rationalize that providing aid that helps groups like AQ and ISIS today is ok while we share common interests and objectives - and that we will be able to deal with whatever consequences come from that later once those interests and objectives once more naturally diverge. I imagine that the hope is that once the dust settles, those that are too radical will be pushed aside by more moderate Sunnis who will ultimately form governments of new states that emerge. Could happen.

4. Lastly, while there are risks to this approach, it is way more feasible, acceptable, suitable and complete than any idea of simply roaring back into Iraq with a large American presence and forcing the old, infeasible, unacceptable, unsuitable, incomplete solution we tried before to finally work.

The bottom line is that one cannot attain any sort of natural stability within any system of governance until one can get to some reasonable degree of trust between the parties within that system. How does one get to trust in modern Iraq or Syria within the confines of those clumsy colonial borders? I don't think one can. A strong leader like Saddam could force an artificial stability (like exists within prisons...), but that is not anything we can create or facilitate. And frankly, even the Saddams, Titos, Stalins, Mubaraks, etc of the modern era find a much more difficult challenge that their preceding role models in the current strategic environment where people are so connected and informed.

I know this will be messy, but I for one, am optimistic that it might actually produce a reasonably durable result that is not overly branded with US ownership.