News media are reporting today that the strife in Syria has, for the first time, spilled across international borders, with Syrian government forces firing into Turkey last night, killing two people and injuring three others, and also firing into Lebanon. The New York Times suggests that a “large number of reinforcements for the government troops, backed by tanks and helicopters,” may have arrived “close to Turkish territory.” And of course Turkey is already sheltering a large number of refugees from the conflict—over 24,000, by the Turkish government’s estimate.

All of which raises the question of what, if anything, can be done. For the past year, the answer has been very little: Russia and China blocked effective measures in the Security Council; the legitimacy of humanitarian intervention on the basis of the responsibility-to-protect (“R2P”) principle has been contested; and neighboring states seemed to lack a persuasive argument for intervention on the basis of self-defense.

But yesterday’s events suggest that the self-defense argument is strengthening. Article 51 of the UN Charter recognizes an “inherent right of individual or collective self-defense if an armed attack occurs against” a member state, “until the Security Council takes measures necessary to maintain international peace and security.” There is at least a reasonable argument that by firing bullets across the border, amassing troops nearby, and forcing Turkey to cope with a significant influx of refugees, Syria is violating Turkey’s territorial integrity and creating justification for an armed Turkish intervention on the basis of a Turkish right of self-defense.

To be clear, I’m not necessarily advocating the legality of intervention; I’m saying simply that the argument for a self-defense-based intervention is getting stronger. And, of course, whether intervention makes sense as a policy matter is another issue altogether.

Cross-posted at PrawfsBlawg.