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?