Internal armed conflicts in the meaning of Common Article 3 are those pursued either between the armed forces of a State and armed non-state groups, or in between such groups.
The International Committee of the Red Cross (ICRC) provides:
Non-International armed conflicts are protracted armed confrontations occurring between governmental armed forces and the forces of one or more armed groups, or between such groups arising on the territory of a State [party to the Geneva Conventions]. The armed confrontation must reach a minimum level of intensity and the parties involved in the conflict must show a minimum of organisation.
The first part of the definition shows two key criteria for determining whether an armed conflict is of internal or international character; the territorial limitations and the limitations as to the parties involved. The territorial limitation in Common Article 3 provides that the conflict must take place in the territory of ‘one of the High Contracting Parties’. A strict reading of this Article leads to the conclusion that the conflict must remain within the borders of one single State, but it is asserted that the Article does not cease to apply just because the conflict spills over to the territory of another State. According to the ICRC, the conflict shall arise on the territory of a State for Common Article 3 to be applicable, clearly opening for the possibility of “spillover” effects into the territory of other States. The spillover of a Non-International Armed Conflict into adjacent territory cannot have the effect of absolving the parties of their International Humanitarian Law obligations simply because an international border has been crossed.
DISCUSSION PAPER 2: DRONE ATTACKS, INTERNATIONAL LAW, AND THE RECORDING OF CIVILIAN CASUALTIES OF ARMED CONFLICT, pp 7-8