Numerous commentators (including yours truly) have condemned President Trump's precipitous withdrawal of US forces from northern Syria as a betrayal of our erstwhile Kurdish allies. It was and remains such a betrayal. Despite yesterday's announcement of a 5-day "pause" in operations--which was predictably and inaccurately hyped by Trump as a "great" deal that resulted from his "tough love"--Turkey apparently has no current plans to withdraw forces from its self-declared "safe zone" in northern Syria.
Thus, Turkey's incursion leaves alive the possibility of clashes between Turkish and Syrian and/or Russian troops. Such clashes in turn might result in a call for NATO involvement. Suppose Syria crosses the border and counterattacks. Suppose Russia, which has troops stationed in Syria about 20 miles south of the Turkish border, assists in such a counterattack. Would that constitute an "armed attack" under Article 5 of the NATO Treaty, thus obligating the US and other NATO countries to come to Turkey's defense?
Given the potentially catastrophic results of such a great-power conflict, one hopes not. Here I'll parse the key documents (the NATO Treaty and the UN Charter) to see whether we can reach that result.
The short answer is, thankfully, no: A counterattack by Syria or Russia on Turkey would not trigger NATO Article 5 because that provision pretty clearly incorporates by reference UN Charter Article 51. Here's NATO Article 5:
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.True, a counterattack by Syria or Russia would be an attack on Turkey and presumably would be carried out by armed forces. In that sense such a counterattack would be literally an "armed attack." However, it is clear in context that NATO Article 5 treats "armed attack" as a term of art. It does so by invoking the right of self-defense recognized by Article 51 of the Charter. The NATO Treaty postdated the UN Charter by less than four years and clearly indicates in its text that NATO nations were committing to a mutual defense pact, not to a mutual aggression pact.
To be sure, we can imagine circumstances in which a NATO nation takes inappropriate action to provoke the use of force against it. One would then need to determine whether the provocation was of such a nature as to vitiate the right to self-defense under Article 51 and therefore to render inapplicable the collective defense provision of NATO Article 5. But in the current circumstances--in which Turkey is clearly the aggressor and we are imagining a counterattack--there is no plausible account of Turkey's continued use of force that fits within Article 51.
Interestingly, if one were to conclude otherwise--that is, if one were to conclude that the NATO Article 5 duty to aid fellow NATO members applied even when the (counter-)attacked NATO member was the aggressor--that duty might be binding notwithstanding the UN Charter. I confess that as a constitutional law scholar accustomed to thinking about the law in nested hierarchies -- US Constitution prevails in conflicts with federal statutes, which prevail in conflicts with federal administrative regulations and state laws, etc. -- I would have thought that in a case of conflict between the UN Charter and a multilateral treaty encompassing far fewer parties, the UN Charter would prevail. However, international law does not generally work like that. Here's a useful 2017 article by Mario Prost both identifying the absence of such hierarchy in international law and also arguing that, as a practical matter, there are hierarchies.
Yet even if we accept the conventional wisdom and say that no treaty prevails over another, we would still have an interpretive canon that aims at harmonizing treaties. And here it is extremely easy to harmonize NATO Article 5 with UN Charter Article 51 by construing "armed attack" in the former as coextensive with its meaning in the latter. Accordingly, the unfolding disaster in northern Syria will not likely include further military involvement by the US or other NATO members -- at least not pursuant to NATO Article 5.