When can citizens of a state be considered responsible for the outbreak of an interstate conflict?

The origin of the norms of international law can be attributed to the period of the primitive communal system, when allied ties began to be established between the tribes, which included, first of all, the interests of protection from external danger. The process of formation of states led to the final formulation of interstate relations and international law.
Disagreements arise between states, as well as between individual citizens, often leading to acute conflicts. A characteristic feature of interstate contradictions is that they have a serious impact on the fate of a large number of people, especially during military conflicts.