The basis of the Nicaragua case: what happens when there is the same rule of law in customary law and treaty law.
1. COEXISTENCE The main conclusion from this case is that the two sources of law can continue to co-exist. Just because a norm also exists in treaty law, does not mean that the source from customary law is not extinguished. For example the law of use of force flows both from treaty law and customary IL.
2. TREATY –> CUSTOM Treaty activity can constitute state practice for the formation of a customary norm. One has to be very clear in analyzing state conduct whether states are only acting under their treaty obligations or whether adherence to treaty obligations has grown to include opinion juris and broader state practice. A good example of this is UNCLOS (UN convention on law of high seas). A lot of UNCLOS provisions have become customary IL. States that are not part of UNCLOS are bound by some of the provisions of the Convention. For a treaty to create custom you need non treaty parties acting as if they are bound by the treaty.
3. CUSTOM –> TREATY custom can be restated in a treaty format. When a treaty codifies customary IL, the treaty is restating customary IL.
4. CONTRACT –OUT A treaty can be used to contract out of customary IL. There are however certain obligations that no state can contract out of i.e. jus cogens norms.