The shoot-down of a Russian jet by Turkey prompts me to review the complicated geopolitics of the Syria-Iraq battle space. Keith Bradsher of the NYT explains what's behind the shoot-down. I'll summarize what he has to say then build out to the bigger picture.
As Bradsher explains, Turkey has been committed to removal of the Assad regime in Syria. So they didn't like it when the Russians intervened on his behalf. They like it even less that the Russians have specifically attacked Turkmen in the north of Syria who are ethnically akin to the Anatolian Turks. Finally, there is a border dispute with Syria which the Russian jets have been irritating. So Erdogan decided to punch Putin.
What he doesn't mention is the situation between Turkey and the Kurds, which is not really related to the dispute with Russia. During most of the time while the U.S. occupied Iraq, Turkey was routinely bombing villages in Iraqi Kurdistan controlled by the PKK, an irredentist movement with designs on Turkish territory inhabited by Kurds. A couple of years ago, however, the Kurdistan Regional Government (KRG) achieved a rapprochement with Ankara. The Kurds repudiated the PKK and suppressed its activities; Turkey granted cultural and political rights to its Kurdish population. Kurds were even elected to the Turkish parliament. This was of course an essential step toward the Kurds' ambition for autonomy, which Turkey would not previously have tolerated. Unfortunately, due largely to Turkish domestic politics, the accommodation between Turkey and the KRG has become rocky.
The Turks are more uncomfortable, however, with the Kurds in northern Syria, where the PKK continues to operate. They don't like NATO's assistance to the Syrian Kurds vs. the self-styled Islamic State, because it means a more capable Kurdish military force on its border, with which Turkey does not have a tolerant relationship. The Turks were very reluctant to allow support for the recapture of Kobani from their territory, and generally reluctant to take on IS. They do now allow the U.S. to operate against IS from their Incirlik airbase, however.
The Baghdad government is also disturbed by the enhancement of KRG military capabilities. Kurdistan long ago seized territory around Kirkuk which has previously been controlled by the Baghdad government, and has now seized Sinjar, and vowed to keep it. So Baghdad has blocked arms shipments to Kurdistan through its airport.
The U.S. and its NATO partners, however, find the Kurds much more reliable partners against IS than the Baghdad army, which is corrupt, ineffectual, and partnered with Shiite militias backed by Iran. The Kurds are also culturally more allied with the West. I should note, however, that the development of the Peshmerga from an insurgent militia to a national army is not complete. Units actually have allegiance to one of the two major political factions that govern Kurdistan. They will fight alongside each other, but rivalries have sometimes slowed military action.
So, while the IS manages to unify Baghdad, Iran, NATO (with some Turkish reluctance), the Kurds, and Russia in opposition, these players do not easily collaborate. Most important, there is no capable Sunni Arab force to take the territory now held by IS, and the relevant population within Iraq will not accede to Baghdad rule, nor will the Syrian population accede to Assad. So there isn't any alternative state to gain their loyalty.
That is why many people are now arguing that a negotiated end to the Syrian civil war is necessary, that will leave the Assad regime in place in control of its rump territory, but create a federal arrangement that gives non-Allawite ethnic groups autonomy, as the Iraqi Kurds have now. There would probably need to be a similar arrangement for the Iraqi Sunni Arabs, and perhaps this even means that the Iraq-Syria border goes away in what is now IS territory. The point is, IS occupies precisely the Sunni Arab territory that will not be governed by the existing recognized nations. Perhaps Iraqi Kurdistan would also expand into what is now Syria, although the Turks probably won't like that. The Iranians aren't happy about increased Kurdish autonomy and power either, as they have their own restive Kurdish population.
But the bottom line is, we aren't getting rid of IS without a political agreement that gets past these fault lines. In the past, we were very reluctant to give up on the vision of a secular, unified Iraq. But that's long gone. Syria and Iraq don't work as states any longer.
Also, and very important, see Gaius Publicus summarizing David Stockman on how the U.S. is principally responsible for bringing about this state of affairs.