An International Armed Conflict exists whenever there is 'resort to armed force between two or more States.'
30 Common Article 2(1) to the Geneva Conventions of 1949 states that:
…the present Convention shall apply to all cases of declared war or of any other armed conflict which may arise between two or more of the high contracting parties, even if the state of war is not recognised by one of them.
The Convention shall also apply to all cases of partial or total occupation of the territory of a High Contracting Party, even if the said occupation meets with no armed resistance.
The Conventions do not provide a definition of either ‘armed conflict’ or ‘war’. War can either be produced by a declaration of war or by the actual and comprehensive uses of force between States. As can be seen in the second section of the Article, there is no requirement that the attacked State use force to protect itself from the attacking State, the comprehensive use of force from one of the States involved is enough for an international conflict to be at hand.
Article 1, paragraph 4 in Additional Protocol I extends the definition of International Armed Conflicts to include wars of national liberation, where people fight in the exercise of their right to self-determination.
DISCUSSION PAPER 2: DRONE ATTACKS, INTERNATIONAL LAW, AND THE RECORDING OF CIVILIAN CASUALTIES OF ARMED CONFLICT, pp. 6-7