Common Article 2 as a paradigmatically challenging threshold for the application of IHL – Rob McLaughlin

Our blog mini-series, co-hosted with the ICRC, wraps up with a post by Associate Professor Rob McLaughlin, Co-Director of the ANU Centre for Military and Security Law.  Dr McLaughlin shares his reflections on common article 2 and the important question of when IHL must be applied.

The key threshold set out in the first part of Common Article 2 (“CA2”) of the Geneva Conventions (“GC”) is deceptively simple:

In addition to the provisions which shall be implemented in peacetime, 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 recognized by one of them…

The most important component of this threshold – given the absence of declarations of war as a formal diplomatic act since World War II – is that of an “armed conflict” between two or more States (the “High Contracting Parties”). If this threshold is met, then an International Armed Conflict (“IAC”) is afoot and International Humanitarian Law (“IHL”) must be applied.

The classic statement of interpretation of this provision, by Pictet in the 1952 commentary, is as follows:

Any difference arising between two States and leading to the intervention of armed forces is an armed conflict within the meaning of Article 2, even if one of the Parties denies the existence of a state of war. It makes no difference how long the conflict lasts, or how much slaughter takes place. The respect due to human personality is not measured by the number of victims. Nor, incidentally, does the application of the Convention necessarily involve the intervention of cumbrous machinery. It all depends on circumstances. If there is only a single wounded person as a result of the conflict, the Convention will have been applied as soon as he has been collected and tended, the provisions of Article 12 observed in his case, and his identity notified to the Power on which he depends.

However, this seemingly clear statement is being challenged by evolving state practice on a number of fronts. What, for example, is the CA2 “status” of incidents at sea, where Coast Guard vessels engaged in competing maritime law enforcement operations in relation to a contested resource or maritime area, collide with each other, or fire at each other? In this short piece, however, I will deal with a different question: What follows from the low threshold for an IAC, in relation to other legal thresholds relating to the application of force?

What follows from the low threshold for an IAC?

The prevailing, orthodox interpretation of CA2 is that as soon as one soldier crosses the border and takes an opposing soldier prisoner, the full suite of GC rules apply. That the level of force required to create an IAC is so low often astonishes those not familiar with IHL; the purpose, of course, is to ensure protections are afforded at the earliest opportunity. Yet the oft forgotten quid pro quo is that crossing the CA2 threshold also creates a permission to employ all of IHL’s many sharper rules, such as the right to seek out, close with, and kill opposing combatants – something that would remain impermissible unless in self-defence outside IHL governed situations. This perhaps mundane point about the double-edged nature of IHL carries with it significant cross-paradigm complications.

For example, whilst a border incursion involving a small group of soldiers clearly activates CA2, therefore creating an IAC between the two concerned states, that same act may well not meet the threshold for an “armed attack” such as to trigger the application of Article 51 of the UN Charter. As the ICJ noted in the iconic Nicaragua (Merits) Judgment, (at para 195, ostensibly in relation to armed bands, but of general resonance):

The Court sees no reason to deny that, in customary law, the prohibition of armed attacks may apply to the sending by a State of armed bands to the territory of another State, if such an operation, because of its scale and effects, would have been classified as an armed attack rather than as a mere frontier incident had it been carried out by regular armed forces. (emphasis added)

That is, whilst a mere frontier incident may well cross the CA2 threshold for an IAC, it is conceivable that it will not simultaneously cross the threshold for an Article 51 “armed attack”. The consequence, of course, is that orthodoxy at the jus ad bellum level would require states to respond to such uses of force that are below the UN Charter Article 51 threshold of “armed attack”, with non-use of force counter-measures, as per the Articles on the Responsibility of States for Internationally Wrongful Acts, Article 50:

Obligations not affected by countermeasures

  1. Countermeasures shall not affect:

(a) The obligation to refrain from the threat or use of force as embodied in the Charter of the United Nations…

For all uses of force that do constitute a UN Charter Article 51 “armed attack”, it is thus inescapable (and entirely proper) that the same factual nexus also mandates that the CA2 threshold has simultaneously been crossed and that the resultant hostilities must be conducted in accordance with IHL.

But what about a situation where the CA2 threshold is crossed, and IHL is thus mandatorily to be applied, but the initial triggering act did not rise to the level of an armed attack? In this case, we are confronted with both a “strategic” prohibition on use of force as a counter-measure, and a clear “tactical” permission – indeed in some cases a requirement, such as to detain as a prisoner of war – to use force in accordance with IHL.


Doctrinally, we must always bear in mind the jus ad bellumjus in bello bivalency through which this dilemma is resolved in a purist legal sense. As the updated GCI Commentary on CA2 notes (at para 125 for example), the validity or otherwise of the jus ad bellum does not affect the application of IHL as the jus in bello. But we must also grapple with the clear implication that the law pulls us in two diametrically opposed directions in such situations. CA2 – as simple as it initially appears – can presage significant complications for law abiding states (and their legal advisers) when it is required to operate in parallel with other equally fundamental norms, where those other norms may actually view the same factual nexus in a radically different way, and as permitting of very different authorisations and consequences.

Associate Professor Rob McLaughlin is Co-Director of the Centre for Military and Security Law, Australian National University College of Law.