The interplay between municipal and international law is complex. Some authors believe that international law and the law of the domestic jurisdiction, also referred to as the municipal law of the country, do not intersect and are completely different entities which cannot affect or overrule each other. However in practice it seems that this does not hold true. There are some principles that are clear as this conflict between international law and domestic law is concerned.
Municipal law cannot serve as a defense to a breach of international law, i.e. you cannot use a domestic law to justify the breach of an international one. Neither can one say that their consent to a treaty has been invalidated by way of a change of its municipal law. Similarly, the International Court of Justice has also stated that the lack of domestic legislation cannot be brought up as a defense if there is an international obligation on the state not to do a certain act. There have been various cases on points that state that international law is prevalent over the domestic law however that does not mean that domestic legislations carry no force.
Some international treaties require that countries adopt domestic legislation in line with the international obligations it has already agreed to. Owing to such requirements there is a blurring of the distinction between international and municipal law and domestic courts have also started analyzing international obligations of states in domestic disputes. In countries such as the United Kingdom, there is a doctrine of transformation that states that before any international agreement can be considered applicable domestically it must be transformed into municipal law. This means that the provisions of the treaty need to be transformed into local law, passing a domestic legislation with concurrent provisions as the international obligations.
Similarly in the United States of America, the position is that customary international law is federal law and if the federal courts in the US determine it to be binding then it's binding on the state courts as well. However, no act of legislature may be invalidated merely on the basis of a violation of customary international law. The US Supreme court believes that there should be respectful consideration to be given to the interpretation of international treaties however a domestic rule to the contrary would be given supremacy over those provisions.
