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And now, only recently, Turkey finally took a position against ISIS, and began to use its own military. Of course, while they bombed some ISIS targets, they also bombed the Kurds in Iraq. This is also when they finally allowed the US to use Incirlik as a base to bomb ISIS.
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I finally figured out the details of how Turkey recently changed it position to let the US use the base at Incirlik for missions inside Syria. And it, of course, involves the Kurds.
The Syrian Kurdish force is called the YPG. This is one of the groups the US supports. In order to change what had been a solely "Kurdish face" of the anti-ISIS ground force in that part of Syria, they aligned themselves with some Arab brigades to form the SDF (Syria Democratic Forces). And they have been successful pushing ISIS away from areas along the border with Turkey.
The SDF is now fighting Turkey-backed factions in the northern countryside of Aleppo. This is a key land area inside Syria (the area west of the Euphrates River, which Turkey considers a "red line"). The SDF had already captured some towns close to the Turkish border (example - Kilis, which is an important supply line from Turkey into Syria).
What the YPG wants is to link all the predominantly-Kurdish villages all the way to the Iraqi border. But, to make this actually happen, they must also take control of another key bordertown -- Jarablous -- from ISIS. Over the summer, there was a planned Kurdish offensive to do just that, with US air support. Taking it would also hurt ISIS by closing one of their remaining supply lines.
This is when the Turks blinked. As I mentioned above, the Turks biggest fear has always been Kurdish autonomy in northern Syria which might stir up similar sentiments among Kurds in Turkey. This was true both before and after the Russian jet was shot down.
So, in exchange for the Kurds not advancing on the Aleppo region, the US got use of the Incirlik air base (inside Turkey) to bomb ISIS.
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There is now one more part to all of this - the Russians. Since the jet downing, Putin not only wants to secure Assad's but to also stick it to Turkey. What Putin seems to be trying to engineer now is unification between the Assad Govt and the YPG. It hasnt happened yet but it becomes possible if Assad were now willing to drive ISIS out. If so, we could be left with a situation where ISIS is driven out of northern Syria and replaced by a secular, democratic swath dominated by the Kurds. Which would probably also be protected as a no fly zone.