[Michael W. Lewis is a Professor of Law at Ohio Northern University where he teaches International Law and the Law of War.]
Kevin was right that my Just Security post misstated the legal standard for self-defense by stating that Syria could rightfully treat US attacks on ISIS on Syrian soil as aggression if the US had established that it was acting in self-defense. As he said, such a use of force in self-defense cures any sovereignty violation that the United States might have committed. This is, of course, how it works in theory. Reality is somewhat different.
In practice, any state relying on the “unable or unwilling” standard (as the United States did in Pakistan to support the bin Laden raid) will have no way of knowing whether the target state will see things the same way. By definition a state relying on the “unable or unwilling” standard lacks permission from the host/target state to use force on its territory. This is why I said that the US would act at its own peril in Syria. Any state taking such action will do so at its own peril because the host/target state might believe itself to be justified in using force to repel perceived aggression. That is why the US used its most advanced and stealthiest helicopters for the bin Laden raid because they anticipated that Pakistan might react to an unannounced incursion with force.
Further, in most incidents of anticipatory self-defense (which is what any strike relying on the “unable or unwilling” standard is likely to be based upon) the host/target state claimed that the use of force on its territory was illegal and in many cases did exercise what it maintained were its sovereign rights to respond to the incursion with force. To use the 1967 War as an example, Israel claimed that its first strike against the Egyptian Air Force was an exercise of self-defense because Egyptian, Syrian, Jordanian and Iraqi troops were massing on its borders and Egypt had closed the Straits of Tiran to Israeli shipping. IF Israel’s claim of self-defense was valid this would cure its sovereignty violations, and the Egyptian anti-aircraft batteries would be prohibited from firing on the Israeli planes as they bombed the Egyptian airfields. Even if theoretically international law contained such a prohibition, would any state in Egypt’s position honor it? The answer is self-evidently, no.
The reality is that any states relying on the “unable or unwilling” standard to support a claim of self-defense will do so while anticipating and preparing for armed resistance from the host/target state. And host/target states which have not granted permission for others to use force on their territory will assert a right to defend their sovereignty by treating such uses of force as aggression, and by responding with force if they so choose. The host/target state’s response, though theoretically unlawful, is very likely to occur and is something that any state relying upon the “unable or unwilling” standard will both anticipate and factor in to its decision to use force.