(176) … article 51 [self-defence] of the Charter is only meaningful on the basis that there is a “natural” or “inherent” right of self-defence, and it is hard to see how this can be other than of a customary nature…. Moreover the Charter, … does not contain any 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 international law.
(186) It is not to be expected that in the practice of States the application of the rules in question should have been perfect, in the sense that States should have refrained with complete consistency, from the use of force or from intervention in each other´s internal affairs. 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. In order to deduce the existence of customary rules, the Court deems it sufficient that the conduct of States should, in general, be consistent with such rules, and that instances of States conduct inconsistent with a given rule should generally be treated as breached of that rule, not as indications of the recognition of a new rule. 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, the 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.
(193) …it is also clear that it is the State which is the victim of an armed attack which must form and declare the view that it has been so attacked. There is no rule in customary international law permitting another state to exercise the right of collective self-defence on the basis of its own assessment of the situation. Where collective self-defence is invoked, it is to be expected that the State for whose benefit this right is used will have declared itself to be a victim of tan armed attack
[Commenting on the incursions from Nicaragua into Costa Rica and Honduras, and the supply of arms to the opposition in El Salvador, and regarding the nature of armed attack, the Court said:]
(195) 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 band 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
(196) 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
[regarding the obligation of reporting self defence action under article 51 UN Charter]
(200) the absence of a report may be one of the factors indicating whether the State in question was itself convinced that it was acting in self defence
(231) there are however other [vg. No declaration of the victim that it was suffering an attack, no request for help to the USA] considerations which justify the court in finding that neither these incursions nor the alleged supply of arms to the opposition in El Salvador may be relied on as justifying the exercise of the right of collective self-defence [see paragraph 195]
(236) the condition sine qua non [armed attack?] required for the exercise of the right of collective self-defence by the United States is not fulfilled in this case
(268) While the USA might form its own appraisals of the situation as to respect for human rights in Nicaragua, the use of force should not be the appropriate method to monitor or ensure such respect. With regard to the steps actually taken, the protection of human rights, a strictly humanitarian objective, cannot be compatible with the mining of ports, the destruction of oil installations, or again with the training, arming and equipping of the contras
(269) …militarization of Nicaragua which the US deems excessive, …an argument to justify its activities with regard to Nicaragua. It is irrelevant and inappropriate, in the court’s opinion, to pass upon this allegation …since in IL there are no rules, other than such rules as may be accepted by the state concerned, by treaty or otherwise, whereby the level of armaments of a sovereign state can be limited, and this principle is valid for all states without exception
(270) having concluded the examination of Nicaragua claims based on CIL, the court turns to claims under 1956 Treaty of friendship. Nicaragua argued that US by its conduct has deprived the treaty of its object and purpose, and emptied of real content. Nicaragua has relied on the existence of a legal obligation of states to refrain from acts which would impede the due performance of any treaty entered into them. Is there a duty of a State not to impede the due performance of a Treaty to which it is a party that is not a duty imposed by the treaty itself. Nicaragua contends that this rule is implicit in pacta sunt servanda.
(275) there are certain activities of the US which are such as to undermine the whole spirit of a bilateral agreement directed to sponsoring friendship between the two states. (direct attacks on ports, oil installations etc)