Until now, Belgium's contribution to the air campaign against ISIS has been limited to strikes on targets in Iraq. This constraint reflected, at least in part, a sense that the legal case for strikes in Iraq (from a UN Charter perspective) was clear (in light of the consent of the Iraqi government), whereas the legality of strikes in Syria (where the Assad regime did not consent) was murkier.
Well, the Belgians have changed their mind. As noted here and here, Belgian F-16s soon will be striking ISIS targets in both places. And it would seem to follow that the Belgians both accept the unwilling/unable concept in principle and have now concluded that it is applicable in the context of the Syrian government's manifest inability to suppress ISIS. I've not yet seen any formal Belgian statements to this effect, mind you, but it is hard to see what alternative argument they would rely upon. At any rate, it could be that this will be another data point relevant to the debate over the legitimacy (and nature) of the unwilling/unable concept. Paging Ashley Deeks!
Apropos of such questions, it is worth noting that the Wall Street Journal piece on this subject also devotes some space to the growing problem of ISIS in Libya, and in doing so it has this to say:
European diplomats have said that the international community must proceed cautiously. The Libyan government has made clear that to survive it cannot immediately invite everyone in to conduct military strikes or training missions. “This is an ongoing discussion, ongoing planning,” said a senior western official. “Everyone underlines the need to be cautious and have a clear request from the Libyan government.”
This is a nice illustration of the way that sovereignty issues involve both legal and practical/political dimensions, and thus also a nice illustration of how one has to be careful not to conflate the two. On one hand, as noted above in relation to Syria and Iraq, having consent from the territorial state is generally thought to be good enough to obviate any Article 2(4) objections. And here we have an unnamed official emphasizing a seemingly-strict need for Libyan consent. But I don't think that necessarily is best read as a legal claim. The point may simply be a practical one: the unstable central government might collapse if too many foreign interventions occur, quite apart from the legality of those questions. And thus, though one might argue that Libya too presents an illustration of "unable" in the context of ISIS, practical considerations will still act as a serious check on the use of force against ISIS there.