- One fighter shot down is not an attack. It is kind of a "defense." It is too small of an event to trigger anything.
- Even if Syria did something more significant, there is no automaticity to a NATO response. The members would gather and vote eventually. NATO members could then disagree about whether anything that happened was attack-y enough. Just as someone will not get tenure because there are enough profs who do not like the candidate, merit has got only a bit to do with this. Turkey has had rough relations with a number of NATO members over EU membership. So, it is not the most favored member of the org. Oh, and you can bet that Greece would be less than helpful.
- Even if NATO agreed to invoke Article V--an attack upon one = attack upon all, this would not produce an automatic response or result. The language of article V provides heaps of wiggle room--each country to help as it deems necessary. So, after Article V was invoked after 9/11, some members of NATO did not participate in the patrols of NATO aircraft over American cities during major events. So, an attack upon Turkey, even if A.5 is invoked, would not actually obligate any NATO member to respond in any specific kind of way.
- With the US exhausted and reluctant to expend more resources on yet another war in the Mideast, it would certainly not be pushing for aggressive response until/unless Syria does something more significant than shoot down one plane. What the US really does not want is for there to be a debate about Turkey at NATO since that would raise tensions and widen existing cleavages within the alliance.
If NATO did act in Turkey's defense, then the UN loses relevance. NATO has acted before without the UN (Kosovo), and the grounds here would be better--collective defense. This does not mean that Russia loses relevance as there are plenty of tools in the Russian box of annoyance, including sending more arms, sending personnel, and otherwise raising the risks of escalation.
As I was asked via twitter, this is not the first Russian friend to face these challenges, so why would Russia be more concerned and more willing to engage in risky behavior over Syria than over other places? I don't really know except order does matter--that is, this is AFTER Libya and after other friends fell. Is this put up or shut up time for Russia? I hope not. Then again, NATO is somewhat at risk here as well.
This Game of Syrian Thrones: very dangerous. But nobody is jumping too quickly into the fray yet.