The principles of supremacy
The principle of direct effect was created by the ECJ in the revolutionary case of  Van Gend en Loos. Effectively it confers direct effect on EC law provisions, provided that they fulfil the criteria of being clear, precise and unconditional. Initially, under article 288 TFEU only regulations were to be “binding entirely and directly applicable in all Member States  ", however the ECJ has extended the principle of direct effect to other provisions in its jurisdiction under Article 267 TFEU. In this case the ECJ referred to the community as representing ‘a new legal order’, giving community law independent legal status and allowing it exercise significant control over Member State’s legislation. The aims of direct effect were to achiever uniformity amongst the member states so as to maximise the effectiveness of community law. However, in the following case of Costa v ENEL this proved difficult when the Italian court claimed that Italian legislation should take precedence because it post-dated the EC treaty. If the ECJ were obliged to validate this claim, the desired effect of the doctrine of direct effect would be severely undermined. “The executive force of community law cannot vary from one state to another in deference to subsequent domestic laws, without jeopardizing the attainment of the objectives of the treaty set out in Article 10." Furthermore, in Simmenthal II  , the court declared that all EC legislation, from the moment of its enforcement, ‘renders automatically inapplicable any conflicting provision of national law". The ECJ judges inferred from Article 4 TFEU and Article 288 TFEU to assert that there had been a transferral of power by Member States to the Community and this required them under Article 4 to “abstain from any measure which could jeopardise the attainment of the objectives of this treaty". Hence, the judges created the principle of supremacy in order to empower the doctrine of direct effect so if a conflict arises between national law and EU law, the latter takes precedence; this is true of all form of national law, even national constitutional law as exemplified in International Handelsgesellschaft  . The relationship between these two doctrines is therefore one of interdependence, they can exist without each other but their effect would be limited resulting in incomplete European integration.
The relationship described above between supremacy and direct effect is assessed by Michael Dougan using two models; the  “primacy model" and the “trigger model". Effectively, the primacy model is concerned with the separate effects of each principle. In simple terms, supremacy without the support of direct effect has only a limited “exclusionary effect", thus it only works when national legislation is in conflict with EU legislation. Direct effect on the other hand is only effective when “substitutionary effects" arise, so in a novel area of law not yet recognised by national law. This model is obviously not sustainable in the system we have today because supremacy and direct effect co-exist in a way that allows strict enforcement of EU law (with minor exceptions). The second model of interpretation is more intellectually sustainable. The “trigger model" identifies that supremacy is in fact dependant on the doctrine of direct effect for the reason that, in order for EU legislation to have either exclusionary or substitutionary effect in a national legal system, it must satisfy the criteria outlined by the ECJ. Though Dougan’s theories on the relationship between supremacy and direct effect are useful, there is limited support from case law to both models. Furthermore, his assertion that the principle of supremacy is effectively dependant on that of direct effect is highly debatable. The ECJ itself highlights the importance of supremacy, “this provision [direct effect], which is subject to no reservation, would be quite meaningless if a state could unilaterally nullify its effects by means of a legislative measure which could prevail over community law."  This statement made in the Costa case seems to suggest the opposite, that as a matter of fact, direct effect is dependent on supremacy.
Direct effect and supremacy are closely related chronologically. Direct effect was established in 1963 in Van Gend en Loos (case 26/62)  whilst supremacy was developed a year later in Costa v ENEL (case 6/64)  . The two cases relate in that “in Van Gend en Loos, the autonomy of the rights and of the legal order which gave them birth predicted the outcome in Costa"  . This can lead scholars to believe that the “supremacy principle is a logical corollary of direct effect"  . Simply, if a directly effective piece of legislation comes into conflict with national legislation, the supreme nature accorded to it (by supremacy) allows it to set aside the national legislation. Since the principle of supremacy was created after direct effect, inferably it suggests that its introduction was necessary to achieve the effectiveness of EU legislation which direct effect had been sent out to do. Cruz sees the principle of supremacy as having “a wider significance, since all community law, irrespective of its direct applicability, is supreme over all national law, constitutional law included"  . Whilst he acknowledges that direct effect and supremacy are ‘clearly related’, he asserts that they have “different practical effects". Whilst this is true, it is important to acknowledge that both principles are used to achieve the effectiveness of community aims and ensure uniformity and neither could function without the use of the other.
One of the most significant ways in which European integrations has changed is through the submissions of sovereignty from member states to the Community. The doctrines of direct effect and supremacy have arguably undermined the sovereignty of the member states for a number of reasons. After the declaration of the primacy of EU law, member states such as Germany and France attempted to challenge this principle so as not to affect their constitutional legal system. In Internationale Handelsgesellschaft  the ECJ were presented with the first challenge to doctrine of supremacy as the German court claimed that a German constitutional right was violated by an EC regulation. The Community, under the doctrine of supremacy upheld the regulation, in turn asserting that community law overrides all member state legislation including constitutional provisions. Hence , it became clear that Community law had absolute primacy and states which wanted to be a part of the European Community would have to surrender a significant part of their sovereignty in order to respect the treaty regulations. This is also exemplified in ex parte Factortame  , a case in which the British courts were asked to grant an interim injunction against the crown, something they had never done before, because of a conflict between EC law and national law. Under Article 4 TFEU, national courts had an obligation to protect the interests of their persons which arise from Community law even if it posed a threat to national legislation. A new shift in the integration process has also come as a natural consequence of the community’s cohesion policy to unite the member states in economic terms as well as legally. The community’s adoption of the policies of direct effect and supremacy has meant that it “now possesses most the characteristics of a federation"  . The community has become “supranational"  in the legal sphere and has achieved unity amongst its member states by requiring that they all adopt and give primacy to directly effective European legislation. As a result any state that enters the Treaty of Lisbon is subject under these regulations making the integration process more efficient and enhancing the effectiveness of the EU’s policies. One way in which the Community uses its supremacy is in sectoral integration in which the EU regulates on new policy areas rather than allowing Member States themselves to legislate in these sectors. This ensures more effective integration process because the policies regular by EU have to be the same in all member states whereas the policies regulated by member states themselves obviously differ from one to another.
The relationship between supremacy and direct effect is a one which is well established at the core of the European Community. The way in which writers refer to these separate doctrines clearly shows that they are of an inter-reliant nature as they are used together to achieve the aims of the European Community and ensure its effectiveness in administering European law amongst member states. Furthermore, the application of these principles also created a change in European integration with an increase in adoption of policies by the European Union in which members states cannot legislate because of the doctrine of supremacy. The application of these principles “once it was institutionalised in national legal systems, the transformation of the European legal system shifted the key interpreters of European Law from national governments and their administrators to legal bodies."  The introduction of direct effect made the adoption of policies which violated or contradicted European law less desirable as the case was more likely to be taken to court and impose penalties. This made European integration run smoother because member states became more willing to comply with the policies and regulations set out by the treaties. In political theory, the European Union now presents a “multi level governance" whereby “supranational institutions – above all, the European Commission, the European Court and the European Parliament have independent influence in policy making"  which under the doctrines of supremacy and direct effect the States must apply uniformly and award primacy to.