I disagree with Geoff and Ken's assumption that "unlawful belligerency" is a distinct international law crime or violation of the laws and customs of war when it occurs in international armed conflicts.
To say that a combatant is an "unlawful combatant" is simply to say that the person does not meet the criteria set out in Art. 4 of GCIII and that this person does not benefit from the privileges of lawful combatantcy. However, failure to meet these criteria is not by itself a violation of the laws of war. What it does mean that the person can be prosecuted for the ordinary crimes which any combatant would otherwise be prosecuted for if not entitled to combatants immunity/privilege. Thus, an unlawful combatant can be prosecuted for murder, destruction of property and other acts of violence which are lawful under the laws of war. Now, if a State were to make it an offence for a person to fight without fulfilling the conditions for lawful combatancy international law does not prohibit that. However, that would not be a violation of the laws and customs of war. It is a domestic crime but one that international law allows prosecution of.
Given what I have said above, it follows that unlawful belligerency cannot be a violation of the laws and customs of war in non-international armed conflict though states are free to prosecute for it. However, more importantly, the concept of unlawful belligerency is redundant in non-international armed conflicts as States are entitled to prosecute any combatant on the other side. There is no combatants immunity in such conflicts so their combatantcy is neither lawful nor unlawful under international law.
Finally, the US Supreme Court in Ex Parte Quirin was similarly confused on the concept of unlawful combatancy when it stated that: “Lawful combatants are subject to capture and detention as prisoners of war by opposing military forces. Unlawful combatants are likewise subject to capture and detention, but in addition they are subject to trial and punishment by military tribunals for acts which render their belligerency unlawful.” While, as stated above, there is nothing to bar a state from prosecuting a person for acts which render his belligerency unlawful, this is not the main point of the concept. The main point is that such a person is subject to prosecution for acts which are lawful under the laws of war.
Dapo Akande, St Peter's College, University of Oxford