An international armed conflict is an armed conflict between two or more states.
Common Article 2 of the Geneva Convention provides that they '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 recognized by one of them. A a declaration of war triggers a state of war and therefore the applicability of the Geneva Conventions even when such a declaration is not followed by armed hostilities. This also serves humanitarian purposes, notably the protection of enemy civilians on the territory of the state that declares war. Rather than 'war', the Geneva Conventions use the term 'armed conflict' to highlight that the determination whether an armed conflict exists within the meaning of Common Article 2 depends on the prevailing circumstances, not the subjective views of the parties to the conflict. Similarly, the lawfulness of the resort to armed force under jus ad bellum does not have any impact on the determination whether or not an international armed conflict exists.
A low threshold
The threshold for an international armed conflict to exist is low: whenever there is resort to hostile armed force between two States, there is an international armed conflict. Contrary to non-international armed conflicts, there is no minimum threshold of armed violence. Such an approach is functional: a single border skirmish between the armed forces of two states or the capture of an individual soldier may amount to an international armed conflict because international humanitarian law contains specific rules that apply to such circumstances.
Use of force by states
For an international armed conflict to exist, the use of force must be by the state: the acts of purely private persons do not constitute an international armed conflict unless a state uses a proxy force. However, this does not mean that states must use their military forces. There are ongoing debates whether the use of force must be reciprocal in the sense that there must be armed confrontations between two states. A contemporary challenge for the classification of armed conflicts is the question whether the use of force by a state against a non-state actor on the territory of another state without the latter's consent amounts to an international armed conflict or whether the armed forces of two states must be involved for an international armed conflict to exist. Albeit controversial, RULAC adopts the first position.
National Liberation Movements
Article 1(4) of Additional Protocol I to the 1949 Geneva Conventions extends the scope of the Geneva Conventions to include armed conflicts in which people are fighting against colonial domination and alien occupation and racist regimes in the exercise of their right to self-determination. There is some discussion whether this means that such conflicts are a form of international armed conflict or rather whether this means that such conflicts are governed by the law applicable to international armed conflicts without being a form of international armed conflict, provided that the state(s) involved are a party to Additional Protocol I.
The scope of the provision is very limited and was aimed primarily at the situation regarding Israel’s occupation of Palestine, the struggle against the Apartheid regimes in South Africa and Zimbabwe (then Rhodesia), and the colonial struggles. Article 1(4) does not cover armed conflicts fought against repressive regimes other than colonial or racist regimes or alien occupation and does not apply to wars of secession. In practice, the provision has never been applied.
International armed conflicts take place between states. In other words, the question whether an entity possesses statehood determines the nature of an armed conflict.
International humanitarian law does not regulate the question whether or under what circumstances an entity possesses statehood. Similarly, it is irrelevant whether the other parties to the conflict recognize the entity in question.
Instead, the question whether a belligerent party possesses statehood is determined by objective criteria under international law. Under the 1933 Montevideo Convention, states are characterized by having a permanent population, a defined territory, a government and the capacity to enter into relations with other states.
International armed conflicts are fought between states, not governments. However, the nature of the conflict may depend on who is considered capable of expressing the will of the state: If a state intervenes in the territory of another state with the latter's consent, there is no international armed conflict. Yet, consent is expressed by a government. Its validity will depend on whether the government is indeed capable of expressing the will of the state for the purposes of international law. Moreover, there may also be changes in government during armed conflicts which may affect the qualification of the conflict.
Irrelevance of recognition
Recognition of governments is only declaratory under international law. When a state resorts to armed force against the effective government, there is an international armed conflict, regardless of whether or not the intervening state recognizes the de facto government. The effectiveness of the government depends on the ability to control the majority of the territory, the capital, and the ability to administer state institutions. For example, the 2001 invasion of Afghanistan was an international armed conflict: the Taliban were the de facto government, although they were not the recognized government.
Conversely, if a state intervenes on behalf of an armed group, recognizing this armed group as the legitimate government will not affect the qualification of the conflict. Otherwise, states could unilaterally change the nature of an armed conflict.
Democratically elected governments
Every government, not only democratically elected governments may consent to an intervention by a foreign state. However, the consent of democratically elected governments carries a presumption of validity: they are capable of expressing the will of the state even in instances where they may no longer be the effective government during an armed conflict. For a thorough analysis of these questions, see M. Milanovic and V. Hadzi-Vidanovic, ‘A Taxonomy of Armed Conflict’ in N. White, C. Henderson (eds), Research Handbook on International Conflict and Security Law, Edward Elgar, 2012, available at SSRN.