Does one country cutting off defensive cooperation with another country constitute a valid casus belli as per international law?

Question

I was reading an article on a developing situation in one of the most volatile regions in the world and the strained ties between US and Pakistan.

It is being speculated that Pakistan might end American access to their air and land space, completely disrupting American ability to supply their troops Afghanistan.

Former US ambassador Richard Olson said:

Since closing Pakistan’s airspace would hinder America’s ability to defend its forces in Afghanistan, Olson, the former ambassador, said the US might regard such action as a “casus belli,” or grounds for war. Other former US officials echoed that assessment.

That seems like a rather confusing assessment. Isn't it a right of a sovereign state to decide who should they cooperate with? Does it really constitute a valid Casus Belli? By that I mean would that make it a lawful war in the eyes of United Nations and international community? Let's ignore the odds of that happening or implications of such a move and focus purely on the legal aspect.

Anglo-Soviet invasion of Iran comes to mind when Allies invaded Iran to secure a route to supply Soviets against the rapidly advancing Germans so historical precedent does exist although from times before UN.

My question is basically, Does refusal to let Country A supply their troops by Country B constitute a legally valid casus belli as per international laws and norms? If my understanding is correct, the only wars considered lawful by the UN are defensive wars or the ones sanctioned by her, assuming of course that UN has the final say in international law.

I emphasize that my interest in it is purely legal, I am not interested at all in the eventuality of this happening, or the background of US-Pakistan issues etc.

Answer 1

You should not, I believe, interpret the statement you quoted as claiming casus belli acording to international law. As is well known, the US frequently attacks and sometimes invades other states in the world. For most of these interventions, and to the best of my knowledge, the US did not even make a case for having casus belli under international law.

Thus, Olson was referring to casus belli by the US' own moral/ethical standards for itself.

Also, I'm no expert on international law but from the little I do know about law: An action, which does not infringe on a party's acknowledged rights under effective law, also does not constitute a wrong committed against said party which might legitimize perpetrating wrongs in retaliation. Thus breaking off a defensive cooperation is no grounds for much of anything, certainly not war. If I'm wrong, then at the most the "dumped" state may take some kind of legal action in binational or international fora (or even in the courts of the state breaking off the agreement).

Edit: Again, not claiming a final word on legitimate declarations of war, we should also note Article II of the UN charter:

The Organization and its Members, in pursuit of the Purposes stated in Article 1, shall act in accordance with the following Principles: ... (3.) All Members shall settle their international disputes by peaceful means in such a manner that international peace and security, and justice, are not endangered. ...

and there's more language buttressing this in the charter, to which the US is a signatory.

Answer 2

No but yes.

No: without any context, the idea that one country revoking a mutual defense agreement being grounds for a war is silly. They are a soverign state and they can decide who they do and do not support.

Yes: not a casus belli, but if one state does something that is going to get a lot of the second states people killed (like leave them stranded among enemies without supplies), it might just decide that their "friends" have decided to switch sides.

Nations are only bound by agreements in so far as they want to be, so despite whatever agreements they have made or whatever the UN decrees, it always comes down to "Yeah? You and what army?". International courts, treaties, and similar mechanisms are used because it's recognized that going to war at every little disagreement is counter-productive, not because they trump a nations right to decide what to do.

IOW: casus belli just means you have a justification that most will consider sufficient something other than a minor disagreement.

Answer 3

The Americans may well consider it to be a grounds for war, or at least may pretend to, to put pressure on Islamabad, but under international law, it is not. That said, deliberately cutting ties with the West might just persuade enough of the UN not to complain too loudly if they were invaded, which is almost the same thing in today's world.

Answer 4

What you quoted specifically talks about closing airspace. If Pakistan tells the US that they're no longer allowed to fly over Pakistan, the US could press the issue by continuing to fly over Pakistan, at which point Pakistan would have to either concede the point or fire on the planes. At that point, the US could make an (albeit shaky) argument that the firing on US planes is a causus belli.