1.1. Letter from Mr. Webster to Lord Ashburton, 6 August 1842
"Undoubtedly it is just, that while it is admitted that exceptions growing out of the great law of self-defence do exist, those exceptions should be confined to eases in which the necessity of that self-defence is instant, overwhelming, and leaving no choice of means, and no moment for delibaration" (copy available from Yale, Avalon Project)
This clear and concise statement, the "Caroline test", provides the classic requirements for the invocation of self-defence. Before determining how the Caroline test applies to reactive self-defence and anticipatory self-defence, I am going to discuss the general prohibition of use of force under international law.
1.2. General prohibition of use of force and exceptions
The use of force is generally prohibited, as expressed in Article 2 (4) of the UN Charter. The right of self-defence as an exception is laid down in Article 51 of the UN Charter:
"Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain peace and security".
Note that "measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council".
The other exception to the general prohibition of use of force, is authorization by the UN Security Council, as Art. 42 UN Charter reads. As the last sentence of Article 51 UN Charter makes clear, the Security Council may at any time take action, if deemed necessary in order to maintain or restore international security.
1.3. Self-defence (Art. 51 UN Charter)
According to the definition as laid down in Article 51 UN Charter, for self-defence to be justified, an attack must have occurred; furthermore, the attack must have been an armed attack. It is not to say that an "attack" as meant in para 74 of the Diplomatic and Consular Staff case of 1980 cannot qualify as an "armed attack", or that the deliverance of arms in para 195 of the Nicaragua case justifies the invocation of self-defence in accordance with Art. 2 (4) of the UN Charter.
When is Art. 51 of the UN Charter applicable? As the International Court concludes, "the most grave forms of the use force (those constituting an armed attack) must be distinguished from other less grave forms", see para 191 of the Nicaragua case.
Assuming that an armed attack (1) has occurred, the requirements of necessity (2) and proportionality (3) must be met (see also: Nicaragua, para 194; Threat or use of nuclear weapons, para 41; Oil Platforms, para 74).
1.3.1. Customary international law
As can be learned from paras 41-44 from the Threat or use of nuclear weapons Advisory Opinion 1996, the submission of the exercise of the right of self-defence to the conditions of necessity and proportionality, is a rule of customary international law. As stated in the Nicaragua case, there is a "specific rule whereby self-defence would warrant only measures which are proportional to the armed attack and necessary to respond to it, a rule well established in customary law"(para 176 Nicaragua).
1.3.2. Reservations by the US do not obstruct the application of customary law (Nicaragua)
The Court does not consider that it can be claimed that all the customary rules which may be invoked, have a content exactly identical to that of the rules contained in the treaty which cannot be applied by virtue of the US reservation. Nor can the multilateral treaty reservation be interpreted as meaning that, once applicable to a given dispute, it would exclude the application of any rule of customary law (para 175). Even if the customary norm and the treaty norm were to have exactly the same content, the incorporation of the customary norm into treaty-law must not deprive the customary norm of its applicability (para 177).
However, the jurisdiction of the Court was limited in the Nicaragua and Oil Platforms cases, for the US had terminated the acceptance of the Court's jurisdiction. It did so by making a "multilateral treaty reservation"; when judging a dispute concerning multilateral treaties, as is the UN Charter, the Court could only apply norms of customary international law.
1.3.3. Armed Activities: use of force only justified within strict confines
What could, thus, be said about the difference between the UN Charter and customary international law regimes on the use of force? One main case where the ICJ could actually apply the provisions of the UN Charter is the Armed Activities Judgment (Congo vs. Uganda). The Court has found that:
"The prohibition against the use of force is a cornerstone of the UN Charter. Article 51 of the UN Charter may justify a use of force in self-defence only within the strict confines there laid down. It does not allow the use of force by a State to protect perceived security interests beyond these parameters. Other means are available to a concerned State, including recourse to the Security Council" (para 148).
1.3.4. State responsibility and the right to invoke self-defence
Art. 51 of the UN Charter does not expressly require that, for the right to to invoke self-defence, an occurred armed attack is attributable to another state. However, the Nicaragua case, the Armed Activities on the Territory of Congo Judgment, the Tadic case and Genocide case have questioned whether or not states can be held responsible for acts by non-state actors.
The effective control test was developed in the Nicaragua case: "For this conduct to give rise to legal responsibility of the US, it would have to be proved that the State had effective control [..]. The Court does not consider that the assistance given by the US [..] are imputable to that State" (paras 115, 116).
In the Armed Activities (Congo vs. Uganda, 2005) Judgment, the ICJ stated that "it did not find the acts by a rebel group could be attributable to Uganda, although providing training and support to a rebel group may violate obligations of customary international law" (para 161, 162). "While Uganda claimed to have acted in self-defence, it did not ever claim that it had been subjected to an armed attack by the armed forces of the DRC. (paras 146, 147).
A looser test than the "effective control" test was applied by the ICTY in the Tadic case of 1999:
an "overall control" test should suffice (paras 120-122 and 131, 132). This "overall control" test was rejected by the ICJ in the Genocide case of 2007 (Bosnia and Herzegovina vs. Serbia and Montenegro), for it stretched too far the connection which must exist between the conduct of a State's organs and its responsibility (para 406).
Can a state invoke the right of self-defence against non-state actors, assuming that their conduct cannot be attributable to a state? That is a controversial question, but not a hypothethical one.
It is rather unsure whether the traditionalist approach in the Nicaragua, Congo and Genocide still applies, when in practice the right to invoke self-defence against non-state actors is no longer categorically rejected.
2. Requirements of necessity and proportionality
The necessity of self-defence has to be determined by applying the "Caroline test", as derived from the letter from Webster to Ashburton: instant, overwhelming, leaving no choice of means and no moment for delibaration. Assumed that an act of self-defence meets the criterion of necessity, there is another condition of importance: the time between an armed attack and the measures taken to exercise the right of self-defence. On the question of necessity, the Court in the Nicaragua case observes that:
"The US measures taken, cannot be said to correspond to a "necessity" justifying the action [..]. First, these measures were only taken several months after the major offensive. [..] Finally, the Court must also observe that the reaction of the US (in the context of what it regarded as self-defence) was continued long after the period in which any presumed attack by Nicaragua could reasonably be contemplated" (para 237).
Proportionality. Although the invocation of self-defence had already failed in the Oil Platforms case of 2003, the Court states in para 77 that:
"Had the Court found that it [the attack] was necessary in response to the Sea Isle Incident as an armed attack [..] have been considered proportionate. The Court cannot close its eyes to the scale of the whole operation, which cannot be regarded as a proportionate use of force in self-defence".
3. Anticipatory self-defence and the Caroline test: an imminent threat?
Anticipatory self-defence refers to the situation where measures are to be taken, although an armed attack by another state has not yet occurred. The Caroline test must again be applied, now to decide whether an imminent threat justifies anticipatory self-defence. The 2005 Report of the Secretary-General of the UN confirms that the existence of an imminent threat has to be assessed along the line of the main requirements "instant and overwhelming" and "leaving no other means or a moment for deliberation".
Is preventive self-defence permitted? The Secretary-General of the UN has stated in 2003 that, regarding preventive self-defence, "This logic represents a fundamental challenge to the principles in which, however imperfectly, world peace and stability have rested for the last fifty-eight years. My concern is that, if it were to be adopted, it could set precedents that resulted in a proliferation of the unilateral and lawless use of force, with or without justification". The conclusion could thus be that permitting preventive self-defence would be incompatible with the object and purpose of the general prohibition of use of force as laid down in Article 2(4) of the UN Charter (and as reflected by customary international law).
3.1. UN Charter vs. customary international law (Caroline test)
While
measures taken in either reactive or anticipatory self-defence would
have to comply with the requirements of necessity and proportionality,
Art. 51 of the UN Charter stipulates that self-defence can only be
invoked after the occurrence of an armed attack.
The Caroline test,
formulating the requirements of customary international law, leaves the
possibility that one invokes a state of necessity if an imminent threat
has been established,
the necessity of the self-defence is instant and a state has no other
means left to it than a use of force. Both the necessity and
proportionality principle imply that states have to confine themselves
to repelling the attack(er)- self-defence cannot be invoked in order to
pursue or impose punishment on the attacker.
4. Collective self-defence
Article 51 of the UN Charter applied to both individual and collective self-defence. A state may use force in the defence of an attacked state, provided that the attacked state has made a request to other states to engage in collective self-defence.
As expressed in the Oil Platforms Judgment, "..The US has not claimed to be exercising collective self-defence on behalf of the neutral States engaged in [...]; this would have required the existence of a request made to the US by the State which regards itself as the victim of an armed attack" (para 51).
In the Nicaragua case, it is set out that "The exercise of the right of collective self-defence presupposes that an armed attack has occurred. It is [also] evident that if the victim State wishes another State to come to its help in the exercise of the right of collective self-defence, it will normally make an express request to that effect" (para 232). There is no rule permitting the exercise of collective self-defence in the absence of a request by the State which regards itself as the victim of an armed attack (paras 199, 200).
