Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America) (Merits, 1986)

It is the single most quoted case in the subject, and it is examinable under three separate heads.

1. Custom: how practice and opinio juris are proved, and what inconsistent practice does to a rule.
2. Attribution in State responsibility: the effective control test.
3. Use of force and self-defence: the customary status of the prohibition, and the conditions of collective self-defence.

After the fall of President Somoza in July 1979 a Junta of National Reconstruction took power in Nicaragua. United States aid was suspended in 1981, and from September 1981 activities directed against Nicaragua were planned. An armed opposition, the contras, was largely financed, trained, equipped, armed and organised by the United States. In early 1984 a United States government agency authorised the laying of mines in Nicaraguan internal or territorial waters at Puerto Sandino and elsewhere; there were attacks on ports, oil installations and a naval base; there were overflights; and in 1983 an agency supplied the contras with a manual on psychological guerrilla warfare.

The United States justified its conduct as collective self-defence on behalf of El Salvador, Honduras and Costa Rica. Because of a reservation to its acceptance of the Court's jurisdiction, the Court had to decide the case on customary international law rather than on the Charter directly, which is why the opinion is so rich on the sources.

1. What are the applicable rules of customary international law on the non-use of force and on non-intervention, and how are they proved?
2. Are the acts of the contras attributable to the United States?
3. Was the plea of collective self-defence made out?

1. The prohibition of the use of force and the principle of non-intervention are rules of customary international law.
2. The acts of the contras are not attributable to the United States, which is nevertheless responsible for its own conduct.
3. The plea of collective self-defence cannot be upheld. The United States violated the principle prohibiting the threat or use of force by the mining and the attacks, and by its assistance to the contras so far as that assistance involved a threat or use of force.

Agreement between the parties is not enough. This is the passage every sources question wants. "The mere fact that States declare their recognition of certain rules is not sufficient for the Court to consider these as being part of customary international law, and as applicable as such to those States." Bound by Article 38 to apply international custom as evidence of a general practice accepted as law, the Court "may not disregard the essential role played by general practice."

The contrast with treaty law is drawn explicitly: where two States agree to incorporate a rule in a treaty their agreement suffices to bind them, "but in the field of customary international law, the shared view of the Parties as to the content of what they regard as the rule is not" "enough. The Court must satisfy itself that the existence of the rule in the opinio juris of States is confirmed by practice."

How much conformity does practice require? Not perfection. "The Court does not consider that, for a rule to be established as customary, the corresponding practice must be in absolutely rigorous conformity with the rule." It is sufficient that State conduct should in general be consistent, "and that instances of State conduct inconsistent with a given rule should generally have been treated as breaches of that rule, not as indications of the recognition of a new rule."

The breach-that-confirms. The most useful sentence in the whole opinion for an essay: "If a State acts in a way prima facie incompatible with a recognized rule, but defends its conduct by appealing to exceptions or justifications contained within the rule itself, then whether or not the State's conduct is in fact justifiable on that basis, the significance of that attitude is to confirm rather than to weaken the rule." A State that pleads self-defence concedes the prohibition.

Where opinio juris was found. In the attitude of the parties and of States towards certain General Assembly resolutions, particularly resolution 2625 (XXV), the Declaration on Principles of International Law concerning Friendly Relations. Consent to such a text is not merely a reiteration or elucidation of the Charter commitment; "it may be understood as an acceptance of the validity of the rule or set of rules declared by the resolution by themselves." The customary principle of non-use of force is therefore not conditioned by the Charter's institutional provisions on collective security or on Article 43 contingents, and stands separately from the treaty plane.

Non-intervention. "The principle of non-intervention involves the right of every sovereign State to conduct its affairs without outside interference"; examples of trespass are not infrequent, but the principle is part and parcel of customary international law. The Court recalled its own words from 1949: between independent States, respect for territorial sovereignty is an essential foundation of international relations.

The Court found the United States had largely financed, trained, equipped, armed and organised the FDN, and that the contra force was at one period so dependent that it could not have conducted its most significant operations without multi-faceted United States support.

That was still not enough. The question was "whether or not the relationship of the contras to the United States Government was so much one of dependence on the one side and control on the other that it would be right to equate the contras, for legal purposes, with an organ of the United States Government, or as acting on behalf of that Government." Despite heavy subsidies there was no clear evidence of control actually exercised in all fields.

Then the test, stated in a single sentence: "For this conduct to give rise to legal responsibility of the United States, it would in principle have to be proved that that State had effective control of the military or paramilitary operations in the course of which the alleged violations were committed." Participation in financing, organising, training, supplying, equipping, selecting targets and planning the whole operation, "even if preponderant or decisive", is insufficient in itself.

The consequence is stated with precision: "the contras remain responsible for their acts, and that the United States is not responsible for the acts of the contras, but for its own conduct vis-à-vis Nicaragua, including conduct related to the acts of the contras."

What is unlawful is recourse to either the threat or the use of force against the territorial integrity or political independence of any State. Of the acts imputable to the United States, the laying of mines in Nicaraguan internal or territorial waters in early 1984 and certain attacks on ports, oil installations and a naval base infringed that principle. The military manoeuvres near the borders, in the circumstances in which they were held, were not found to constitute a threat of force.

On collective self-defence the Court laid down a requirement that is often forgotten: "in customary international law, whether of a general kind or that particular to the inter-American legal system, 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." The requirement of a request is additional to the requirement that the victim State should have declared itself attacked.

• For a custom question, the four propositions are: declaration alone is insufficient; practice is essential; conformity need not be rigorous; and a plea of exception confirms the rule.
• For an attribution question, state effective control and contrast it with the mere dependence or general control that was proved and held insufficient. Note that the responsibility that remains is for the State's own conduct.
• For a self-defence question, remember the two cumulative requirements in customary law: a declaration by the victim, and a request by the victim.
• Cross-refer to the ILC Articles on State Responsibility, Article 8, which attributes conduct carried out on the instructions of, or under the direction or control of, a State. The Court's gloss on what that control must amount to is effective control.