Does international law hold an Army accountable for killings done by other groups during a military attack?

Question

Consider a situation where a country is partially occupied by another. An army unit strikes across a border town of the occupying force. Their military objective is to destroy an enemy defensive unit, and then retreat back. They do that very successful, and unexpectedly their attack also triggers the collapse of the whole defence of the town. An angry mob uses this opportunity to breach the borders and enter the border town in large numbers. The mob indulges in rioting, looting and killing, taking out their anger at civilians of the occupying country. The army unit had never prepared for this scenario, and choose to ignore the actions of their civilian country men and focus on their military objective.

Under such a scenario, will the Army unit be culpable for the possible war crimes committed by a mob of their countrymen who are civilians? Note that the army hadn't gone to occupy the town. And, realistically, all they could have done is only to partially restrain the mob, without violence, and they were outnumbered.

Answer 1

Responsibility for war crimes is individual. The army commander might be held responsible if the civilians were acting as his proxies under some later traceable, even if not obviously overt command chain. However, this is pretty hard to prove, as some ICTY cases have shown.

OTOH, the political leadership of that country/group can well be on the hook for instigation based on their speeches etc., if they encouraged such acts. If the army commander made similarly widely heard declarations, he could be considered to have been similarly influential. That's because:

Unlike "ordering" and "superior responsibility," instigation does not require a hierarchical relationship between the parties.

Somewhat more subtly, one can be an instigator "by act or omission". I'm alas not aware of case law where the latter came into play prominently, in a scenario like this. (It has been held more narrowly for when "the commander has created an environment permissive of criminal behaviour by subordinates". However civilians would typically not count as belonging to the latter group.)