Why has the supremacy of EU law proved so important for the process of EU integration?
The original Treaty of Rome did not include any reference to EEC law, which today is known as EU law, being supreme over national law. The purpose of the principle of supremacy is to ensure that EU law is uniformly applied and enforced in all Member States and it prevents Member States from acting contrary to the obligations within the Treaty. The principle of supremacy is therefore a judicial development and the commitment of the Court of Justice to upholding this is the single most identifiable reason why EU law has secured sophisticated and deep integration between the Member States. Accepting the supremacy principle has proved problematic for all Member States. Cases C-11/70, Internationale Handelsgesellschaft  ECR 1125 and C-92/78 Simmenthal SpA v Ministero dello Finance  ECR demonstrate the conflict that national supreme courts have encountered when reconciling obligations of protecting fundamental rights under EU law and guaranteeing domestic constitutional provisions. Even domestic constitutional provisions operate subject to the supremacy of EU law.
The principle of supremacy can be explained in the following terms. In situations of conflict between EU law and national law, EU law must prevail. The simplicity of defining the concept hides a rather more complex relationship that exists in the interaction between national law and procedure and the Court of Justice. Supremacy is now largely taken for granted and the Court of Justice has established a clear body of case law which provides guidance to national courts, regarding their obligations to protect individual rights under EU law. The origins of the Court’s commitment to the supremacy principle can be traced to the decision of Case 26/62 Van Gend en Loos  ECR 1 in which the Court stated that the EEC Treaty had ‘created a new legal order’ which, in international law terms, was different both in substance and effect to other treaties, for example the ECHR. The Court stated that the EEC Treaty was intended to:
“..confer rights upon individuals which became part of their legal heritage.”
Though not stated expressly by the Court, this declaration would only make sense if EU law were supreme. The Court expanded this logic in Case 6/64 Costa v ENEL  ECR 585. This case is important because it demonstrates how persons who are often referred to as ‘opportunistic litigants’ have utilised EU law rights. This group can be defined as individuals who enforce EU rights before the Court of Justice to an extent which may not have been anticipated by the Treaty makers.
In Costa v ENEL, Costa was an Italian lawyer and small shareholder in Edison Volta (an Italian electricity company) sought to challenge a 1962 law which nationalised the electricity production and distribution industries. He refused to pay a bill of less than 2 Euro and was brought before the lowest court in Italy who referred the case to the Court of Justice for a preliminary ruling under what is now Article 267 TFEU. The Italian government was adamant that no reference was necessary in such circumstances and that the obligation of the Italian court was to apply national law. The Court of Justice rejected this view and stated that any national court is entitled to seek an interpretation on the application of EU law from the Court of Justice:
“The integration into the laws of each Member State of provisions which derive from the Community, and more generally the terms and spirit of the Treaty, make it impossible for the States, as a corollary, to accord precedence to a unilateral and subsequent measure over a legal system accepted by them on a basis of reciprocity. Such a measure cannot therefore be inconsistent with that legal system. The executive force of Community law cannot vary from one State to another in deference to subsequent domestic laws, without jeopardising the attainment of the objectives of the Treaty (emphasis added)...”
By the reference to the ‘terms and spirit of the Treaty’ the Court goes beyond merely stating that EU law takes precedence over national law. The intention of the Court is, through the use of teleological interpretation of the principle of solidarity included in Article 4 (3) TEU, to demonstrate a dynamic interpretation of the Treaty. European law is to be considered as a new legal order which has a broader objective of integration. The Court places itself at the centre of this integration process and confirms that it will strike down measures which undermine the principle of supremacy and protect the rights which are available under the Treaty.