When examining the relationship between international law and municipal law, it is important to analyse the clash between dualism and monism. Both concepts entail the concurrent existence of international and domestic law.
The question to be assessed is the nature of the co-habitation of these legal orders. Is there a legal order which supersedes the other? Or do they exist cooperatively and non-contentiously? Under the dualism doctrine, a clear distinction is created between international and municipal law, establishing them as separate legal orders which regulate different subjects. Thus, while international law involves the regulation of the relationship between sovereign states, domestic law confers rights to persons and entities within the sovereign state.
It is therefore important to point out that under the dualism doctrine, neither legal order has an absolute, undeniable power to create, alter or challenge the rules of the other system. In that regard, the use of international law in domestic courts can only be allowed through an instrument in municipal law which confers rights to that effect. According to the dualism principle, in a case of conflict between municipal and international law, the domestic courts would apply the former.
In contrast, monism asserts the supremacy of international law within the municipal sphere and describes the individual as a subject of international law. The doctrine is established when international and municipal law form a part of the same system of norms which are based on general notions of fairness. The latter concept somewhat translates into an alternative theory which entails that international and municipal law are superseded by a general legal order which rests upon the rules of natural law.
In the eyes of the monists, the state merely represents an amassing of individuals who are subjects of international law. Although monism clearly possesses a logical and equitable basis (since it estops states with higher capabilities from imposing their own legal rules as the highest and most sophisticated authority) it is submitted that this doctrine directly contradicts established legal rules. For instance, with respect to the position of states, the law recognises that economic entities such as corporations possess a legal personality. In that regard, to claim that sovereign states do not have a legal capacity would not only deprive international law of its primary purpose (to regulate the relationship between states) but would also overtly undermine the doctrine of separation of powers, which is a fundamental part of contemporary democracy.
The state merely represents an amassing of individuals who are subjects of international law.
When discussing the methods of coordination between municipal and international law, academic views challenge the presumptions drawn by the followers of monism and dualism that the two legal orders share a common field of operation. In Sir Fitzmaurice’s view the two systems work in different spheres. This affords them an equal degree of supremacy and precludes them from entering into conflict. When a state does not act in accordance with international law it is not a question of conflict of laws but rather a conflict of obligations. As such, the consequences will relate to that state’s position on the international political scene, but will not, prima facie, undermine the validity of its internal laws.
With regards to the relation between the states’ obligations and municipal law, the legal position is unambiguous. A state cannot use provisions of its own law as a defence to a claim against it for alleged breaches of international law. This rule is exemplified in the Alabama Claims arbitration where the United States was awarded damages against Great Britain for the latter’s breach of its obligations as a neutral state during the American Civil War.
In the Free Zones case it was decided that ‘France cannot rely on its own legislation to limit the scope of its international obligations’. Furthermore, in the Advisory Opinion in the Greco-Bulgarian Communities case it was stated that ‘it is a generally accepted principle of international law that in the relations between Powers who are contracting Parties to a treaty, the provisions of municipal law cannot prevail over those of the treaty’. If a state has signed to a treaty and its domestic laws violate any provisions of that treaty, the state must change said laws in order to fulfil its international obligations.
Due regard must be paid to the decisions of municipal courts as they provide jurisprudential guidance on the effect of the particular domestic law. The relationship between international law and municipal law should be viewed as one of cooperation and symbiosis. As such, international law should recognise doctrines and concepts created by municipal law. The practical implications of this argument arise when considering the admissibility of municipal courts’ decisions in the International Court of Justice (ICJ). In the Brazillian Loans case the Permanent Court of International Justice (the predecessor to the ICJ) decided that due regard must be paid to the decisions of municipal courts as they provide jurisprudential guidance on the effect of the particular domestic law in the municipal sphere. Although, in accordance with the Court’s jurisdiction, international law is primarily applied, it will nevertheless be logical to assume that parties will rely on provisions of municipal law as part of their arguments. As such, they must present said laws in the form of evidence before the court.
As part of the continuous evolution of international law, the ICJ must recognise concepts created by municipal law which historically have had effect on international relations. Thus, where legal issues arise concerning a matter which is not covered by international law, reference will be made to the relevant rules in municipal law. In such cases the court cannot blatantly disregard municipal law as there are no relevant provisions of international law which can be applied.
In conclusion, by examining the relevant academic principles and case law, one can infer that the generally accepted view describes that international and municipal law are supreme in their own spheres. However, one can also argue that there has been a fusion of the operating fields of both concepts. In the spirit of modernisation, both the municipal and international courts have recognised the need to resort to the other’s sphere of operation as aids to interpretation. Moreover, as you will read in the second part of this feature, the English courts have recognised the confinement of the fundamental doctrines of Parliamentary Supremacy and stare decisis. In that regard, although the segregation barrier between the municipal and international sphere remains existent, it is no longer infrangible.
 H. Triepel; Völekerrecht ud Landesrecht (1899); id., p.1
 I. Brownlie; Principles of Public International Law (7th edn; Oxford University Press, Oxford, 2008) p.32
 Oppenheim, International Law, Vol 1 (8th edn., 1955), p. 37
 H.Kelsen; General Theory of Law and the State (Harvard University Press, Cambridge, USA, 1945), p.363-80;
 H. Lauterpacht, Private Law Sources and Analogies of International Law (Longmans, Green, and Co, London, 1927), p.58; S.Engel; H.Kelsen; Law, state and international legal order: essays in honour of Hans Kelsen (University of Tennessee Press, Knoxville, TN, USA,1964), p.308-16
 Salomon v Salomon & Co. Ltd  AC 22; recognised in international law in Barcelona Traction, Light and Power Company Ltd Case (Belgium v Spain) (Second Phase) ICJ Rep 1970 3
 G. Fitzmaurice, 2 General Principles Law, 92 Hague. Recueil (1957, II), p.68-94
 G. Fitzmaurice, 2 General Principles Law, 92 Hague. Recueil (1957, II), p.69
 C, Rousseau, Droit International Public (1953), p.10-12
 Article 27 Vienna Convention on the Law of Treaties
 (1872), Moore, Arbitrations, I. 653
 Case of the Free Zones of Upper Savoy and the District of Gex (France v Switzerland) (1932), PCIJ, Ser A/B, no. 46,p.167
 (1930), PCIJ, Ser.B,no.17,p.32
 Certain German Interests in Polish Upper Silesia (Polish Nationals in Danzig) Case, PCIJ Ser A/B (1932) No. 44,24
 Brazillian Loans Case (France v Brazil) PCIJ, Ser A, No. 21 (1929) 124
 Barcelona Traction, Light and Power Company Ltd Case (Belgium v Spain) (Second Phase) ICJ Rep 1970 3, para 38
 Barcelona Traction Case (supra) para 50