At what point does foreign interference in a civil war trigger law on international armed conflict?

Question

Recent international humanitarian law such as the Geneva conventions draw a sharp distinction between international armed conflict (IAC) and non-international armed conflict (NIAC). See for instance How is the term "Armed Conflict" defined in IHL? by the ICRC.

As I understand it, the main motivation for this is that states have agreed to provide protection to the armies of other states that they may be unwilling to extend to rebel groups in a civil war. So they have negotiated two tiers of treatment. Defining a conflict as one or the other has some bearing on how the war can be legally conducted.

However, since the Nuremberg Principles and the UN Charter established the crime of aggression, it seems that this distinction becomes important for other reasons. There is no international analogue to aggression for starting a civil war as it would fall under the domestic jurisdiction of the state. Therefore Chapter VII doesn't apply and the UN is much more limited in its ability to respond.

But what makes a war "international"? It is difficult to think of a conflict that was not in some way influenced by other states, somewhere along the spectrum from media support, diplomatic cover, supplying weapons, sharing intelligence, through training forces, deploying troops and military hardware, all the way to directing or fabricating the existence of a proxy force from abroad.

My question is, what do international treaties and case law have to say about where this line is drawn? What type or level of interference by other states changes the character of a civil war into an international armed conflict?

Answer 1

What makes a war international?

War is international. If at least 2 countries are involved, we speak about war. Otherwise, we have civil wars, rebellions, terrorists, criminal groups etc. The "war on terrorism" is quite an unfortunate buzzword, used by politicians, not by lawyers.

In theory, the distinction is quite straightforward. If your military is attacking another country (no matter if the targets are military, or it is infrastructure etc.) you're waging war. If you're just selling weapons, you're not. But if your soldiers are going on vacations and end up supporting rebels in some other country, the things are more complicated. The best example is Ukraine.

Can you go further that Russia on Ukraine and still not trigger an international conflict? Well, you can, we've seen that in Serbia. NATO has attacked another country, taken away a big part of its territory, even bombarded Chinese embassy, and everything was said not to be war, but international humanitarian intervention.

The human rights are the part of the United Nations system, and the humanitarian interventions per se are widely recognized, for example the First Gulf War was accepted by the UN Security Council. However, the situation is complicated when only a group of countries agree on such intervention, such as in Kosovo - for Russia and China it was an aggressive war.

There are no clear definitions, because the world powers doesn't agree in that matter. And even with the definitions we have, they have still different interpretations.