This week, the North Atlantic Council (NAC) will meet at Turkey's request to discuss what NATO should do in response to Syria shooting down a Turkish F-4 last week. The short answer will almost certainly be: not much.
The emergency session was called under Article 4 of the North Atlantic Treaty, which provides that "the Parties will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the Parties is threatened." It is a mandatory precursor for action under Article 5, the cornerstone provision of the alliance, which declares that "an armed attack against one" ally "shall be considered an attack against them all."
Some rabid speculation in the Turkish media and elsewhere has raised the specter that the incident will indeed lead not only to this provision being invoked but also to a NATO military response in Syria. That is incredibly unlikely.
Article 5, while relatively short, is packed with caveats:
The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.
Any such armed attack and all measures taken as a result thereof shall immediately be reported to the Security Council. Such measures shall be terminated when the Security Council has taken the measures necessary to restore and maintain international peace and security.
Some commentators have argued that Article 5 is not triggered because the incident didn't take place in Europe or North America and was aimed at an aircraft, not the territory of a NATO member. But Article 6 dispels both of those issues:
For the purpose of Article 5, an armed attack on one or more of the Parties is deemed to include an armed attack:
—on the territory of any of the Parties in Europe or North America, on the Algerian Departments of France (2), on the territory of or on the Islands under the jurisdiction of any of the Parties in the North Atlantic area north of the Tropic of Cancer;
—on the forces, vessels, or aircraft of any of the Parties, when in or over these territories or any other area in Europe in which occupation forces of any of the Parties were stationed on the date when the Treaty entered into force or the Mediterranean Sea or the North Atlantic area north of the Tropic of Cancer.
So, aircraft are specifically included as a potential trigger. And the area surrounding Turkey is included as well; it was added to the original treaty by a 1951 protocol on the accession of Greece and Turkey. Indeed, there would have been little benefit to Turkey in joining NATO if it weren't included under the Article 5 umbrella, the most fundamental alliance commitment.
Instead, the operative phrase that almost certainly disqualifies this incident from an Article 5 response is "armed attack." This is a term of art in international law. One short definition is: "A use of force intentionally directed at a state which, by virtue of its scale and effects, is of such gravity as to justify a responsive use of force by that state in self-defence; contrast with mere frontier incident."
Turkey was engaged in aggressive action along its border with Syria during a particularly tense situation, and depending on whose account you believe, either flew into Syrian airspace or remained in international airspace just outside of it. While shooting down the plane was almost certainly an overreaction—the Assad government has said as much—it's hardly an "attack."
Ultimately, like the "high crimes and misdemeanor" threshold for impeachment set forth by the U.S. Constitution, it's a judgment call. In the former case, the House of Representatives makes the call; in the latter, it's the North Atlantic Council.