Of course, neither Syria nor Russia is going to like backing down on this direct military invasion against a sovereign state. This is exactly the kind of exceptionalism for America and its allies that the Russians, since Libya, have been determined to resist. They will be asking themselves: "If not here, where? If not now, when?" No matter how limited, the Turkish invasion will weaken Syria, and, if allowed to stand for even a short time without military cost to Turkey, will likely mean Turkish control of this region for a long time to come.
The chances of any military reaction, if Turkey keeps to the Kurdish areas, are also extremely unlikely because Putin is smart enough to know that, whatever happens to Syria, this is very quickly going to turn into a liability for Turkey, for the United States, and for Erdogan personally.
There's the international embarrassment for Turkey and the U.S. for having invaded another sovereign county, though American leaders are likely confident they can control the media spin on that--in the West, at least. (The irrelevant dust of Crimea will be frequently thrown in the world's eyes.)
For Turkey, the real cost is going to be the burden of a war against, and occupation of, a resistant and organized Kurdish population. Erdogan's post-coup honeymoon will end fast and nasty. There will be more and fiercer acts of resistance by Kurdish nationalists, more attacks on Turkish interests, and increased political opposition. All the weaknesses that were simmering beneath the coup will boil over again in unpredictable ways. In reaction, Erdogan will undoubtedly become more autocratic than he already is, which won't help his relations with Europe, or the image of NATO.
For America, too, for all the reasons mentioned above, it will become increasingly difficult to hide how completely phony are all of the ostensible reasons for its war on Syria-- a threadbare quiltwork of excuses that tries to cover a rats' nest of bad actors. It will also mean another instance of support for "occupied territory" in the Middle East, and for an ever-more-obvious autocrat in a country with serious, organized political opposition. America's various allied Kurdish organizations will split--those the Americans buy off against those that maintain a principled stance. The U.S. will be paying various high prices, over and over again, to maintain something resembling a passable situation in the occupied Kurdish territories and in Turkey. Good luck with that.
Those are the considerations if Erdogan sticks to the minimum program. His second option is to go all in with the American assault on the Syrian government, and push on to strategic objectives like Aleppo, directly attacking Syrian forces. Even if, as he and the Americans would likely arrange, he avoids initial contact with Russian forces, Erdogan would be getting in Russia's face in a way that Putin would be less likely to ignore. The U.S. would like nothing more than to force Putin into attacking Turkish, rather than American, forces, so it can portray its own response as the defense of a weaker ally. Getting Iran involved, and obliterated, would be even better.
Pursuing this option would be enormously dangerous. The costs to everyone, including the costs to Erdogan and the people of Turkey and the U.S, would be magnified exponentially.
The Syrian stage--one of many--is filling with players fed up with American regime-change arrogance. Iran is a player here, and knows it's the next target, so it will be even less inclined to abide any extension of this invasion. And Iran has as much right to be in Syria as Turkey does. China is seeking closer military ties with Syria and already has military advisors " on the ground in Syria." Somewhere, at some time, some countries are going to decide it's time to resist American exceptionalism, with whatever level of force they can muster. A scenario in Syria where Iran commits significant ground forces, with Russian air, and Chinese logistical, support is not impossible. So, downing Russian aircraft and sinking Chinese ships? Then we are really in unchartered, dangerous territory, facing a risk of regional or world war, including a nuclear conflict.
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