nowadays the European Union has such a strong Court of Justice
The aim of this paper is to provide an explanation why nowadays the European Union has such a strong Court of Justice (the Court). The paper is composed of four main parts. In the first part we will present a brief overview of the evolution of the Court of Justice through time. The second part deals with the creation of the legal doctrines as a means of empowerment of the Court. In the third part we will discuss the cooperation between the national courts and the Court, and finally the last part is dedicated to the relations between Member States and the Court. It should be noted that in each part of the paper we will bring arguments and various examples to develop a structured and complete answer.
The Court of Justice through time
The European Court of Justice (Court of Justice under the Treaty of Lisbon  ) was created in 1951 as the judicial body of the European Coal and Steel Community.  It is based in Luxembourg and is composed of 27 judges, one from each Member State. They are appointed by their Member States for a renewable period of 6 years. The judges are assisted by eight Advocates-General whose main task is to deliver legal opinions.  At the beginning the Court of Justice had only three limited functions:
To ensure the administrative compliance of the Member States with the rules of the treaties,
To resolve the disputes in case the EU laws are vague,
To keep the Commission and the Council of Ministers from exceeding their authority. 
Yet, across time it has managed to become from a marginal political actor one of the strongest and influential political and judicial institutions in Europe. 
According to the Treaty of Lisbon, the Court of Justice “shall ensure that in the interpretation and application of the Treaties the law is observed".  The functions of the Court are as follows:
Infringement proceedings against Member States - If a Member State has failed to comply with the EU law, the Commission can bring infringement proceedings and, if necessary, refer the matter to the Court.  While, a Member State as well can bring an action against another Member State in front of the Court  , but they almost never do that, they prefer to do it via the Commission, which acts as an “arbiter in this situation". 
Judicial review of the EU’s legislative and executive acts 
Preliminary rulings - In case the national courts have any doubt about the interpretation or validity of EU law provisions, they may ask the Court for preliminary ruling. It is mainly done to ensure the uniform interpretation of the EU law. 
The power to interpret the EU law allowed the Court to issue important and sometimes tough rulings such as those about the removal of the national non-tariff barriers to trade and the illegal subsidization of the national industries. The Court went even further in its rulings by entering some policy areas that are particularly domestic as environmental and social policies, gender equality, competition policy, etc.  Moreover, the Court had an influence over the EU policy on such issues as who will represent the Member States during the negotiations for international agreements or if they can enter into bilateral trade agreements with the third countries. 
Taking into account this wide range of the influence of the Court, it is to be questioned whether the Member States could expect that the Court, using its competence to interpret the EU law, could change the EU legal system and make bold rulings that undermine considerably their sovereignty and national interests.  Furthermore, could they suppose that the Court will become a tool for the domestic actors to challenge their national policies? These are powers that the Court created for itself without taking into consideration Member States’ will and intentions. 
Direct effect and Supremacy of the EU law
Pollack has described the relations between the Member States and the Court of Justice by the principal-agent theory. This theory states that principals have interests that are different from those of agents: principals want to have control on the agents, while the agents want to have as much autonomy and authority as possible. 
In fact, the Court of Justice, the agent, has shaped its authority gradually by taking important decisions, namely those establishing the principles of direct effect and supremacy of the EU law. These two doctrines are not included in the treaties; as Mancini says, they are the products of the “judicial creativity" of the Court.  The principle of direct effect (in the case of Van Gend en Loos  ) defines that the EU law not only imposes obligations on the individuals, but it confers upon them rights that they can claim in front of their national courts.  Whereas, according to the principle of supremacy (in the case of Costa v. ENEL  ), if a conflict arises between a provision of the EU law and the national law, the EU law takes precedence.  These principles allow the individuals to rely on the EU law before their national courts, and the latter to refer those cases to the Court by means of preliminary rulings. 
It is fair question to ask how the Court managed to escape the control of the Member States and develop these legal doctrines which seem to be the cornerstone of the development of the Court’s power. Whereas, the same principles put under question the sovereignty of the national governments and parliaments. 
According to Alter, politicians and judges have different time horizons: politicians have shorter time horizons; they are focused on how to manage to stay in office and to avoid the decisions that could affect public policies or bring important financial and political costs. They simply ignore the long-term effects of their actions.  While the Court, taking advantage from politicians’ short-term horizons, managed to expand its authority by establishing its legal doctrines. What is worth mentioning is that the Court didn’t apply those principles immediately to avoid the rise of political concerns or important material impacts.  As Trevor Hartley pointed out:
A common tactic is to introduce a new doctrine gradually: in the first case that comes before it, the Court will establish the doctrine as a general principle but suggest that is subject to various qualifications; the Court may even find some reason why it should not be applied to the particular facts of the case. The principle, however, is now established. If there are not too many protests, it will be re-affirmed in later cases; the qualifications can then be whittled away and the full extent of the doctrine revealed. 
The Court’s case law not only contributed to its empowerment, but it acted as a veritable “catalyst in the integration process"  , especially in the situations when the European integration seemed to be blocked. Mancini argues that the Court of Justice has tried to make a real “constitution for Europe" by using its case law.  One of the areas where the Court’s decisions were crucial is the single market (the four freedoms: freedom of circulation of goods, services, people and capital).  It is difficult to imagine the integration of the single market without the bold rulings of the Court. In the famous case of Cassis de Dijon  the Court issued one of its most influential rulings that suggested the principle of mutual recognition. If the goods are lawfully produced and marketed in one of the Member States, they can circulate freely in any other Member State.  This decision was a significant step towards a more integrated market.
Moreover, it is possible to argue that if the EU didn’t have a strong Court which is actually the institution that makes the EU law work, or without the principles of direct effect and supremacy laid down by the Court, the Member States would act according to their national legislation. This could have an important impact on the functioning of EU key policies and it can affect mainly the single market which will not work properly. Any Member State would be able to protect its own national market by passing national laws that contradict the single market law. This will be a source of state protectionism and discrimination.
The Court of Justice empowered by the National Courts
One of the reasons that explain why the European Union has nowadays such a strong Court relies on the relations between the Court of Justice and the national courts. Weiler has argued that the Court had pursued a policy of mutual empowerment towards its national judicial counterparts. 
When the national courts admit the direct effect and the supremacy of the EU law, they start using the preliminary ruling procedure, gaining in this way powers that they didn’t possess before. It’s particularly the power of the judicial review when the national courts get the right to assess whether their national legislation is compatible with the EU law. 
Weiler points out that the lower courts benefit the most from this cooperation, as they gain powers reserved only to their national high courts before.  Whereas the national high courts are less willing to make preliminary rulings as they don’t want the EU law “to encroach in their legal domain". 
As to the Court of Justice, it benefits substantially from this collaboration. The support of the national judiciaries and, especially the lower courts as enforcers of the EU law, made the Court one of the strongest courts in Europe allowing it to expand the scope of its authority and political power in different policy areas. 
First, taking into account the fact that the individuals can rely on the EU law in front of their national courts, they can even go further by challenging some national policies that the Member States or the Commission will never do.  These issues are politically too sensitive (for instance, the social security rules for the migrant workers in Germany  ) and are exclusive fields of domestic policy. Therefore the governments will never want the Court to have an influence over these policies.  As a result, with the support of individuals and the national courts, the Court of Justice will be able to rule on these specific issues expanding in this manner the scope of its jurisdictional authority.
Secondly, in the Treaties the preliminary ruling procedure is defined as a means of uniform interpretation of EU law  , but the rulings of the Court allowed to develop this procedure into a mechanism of indirect judicial review of the national legislation for its compatibility with EU law.  This was apparently another way for the Court to enlarge the scope of its power.
And finally, the national high courts empowered considerably the Court. In spite of the fact that they are willing to limit as much as possible the referral to the Court, in some cases they themselves prefer to do that. This occurs principally when they want to avoid the responsibility or the criticism of their national governments or parliaments on some politically sensitive issues. In this case they prefer to rely on EU law, thus contributing to the enforcement of the Court’s political power. For instance, Imelda Maher argues that the House of Lords made referrals to the Court to avoid criticism when it made constitutional changes that affected the parliamentary sovereignty. 
The Court of Justice and the Member States
Our sovereignty has been taken away by the European Court of Justice. It has made many decisions impinging on our statute law and says that we are to obey its decisions instead of our own statute law…. Our courts must no longer enforce our national laws. They must enforce Community law…. No longer is European law an incoming tide flowing up the estuaries of England. It is now like a tidal wave bringing down our sea walls and flowing inland over our fields and houses - to the dismay of all. 
Indeed, the Court of Justice managed to avoid the Member States’ control, to affect their sovereignty, to maintain its authority and even is able to gain more powers. It relies on the fact that the decision-making at the EU level is incredibly difficult. Already in 1960s according to the former Prime Minister Michel Debré, Charles de Gaulle, realizing the political threat of the Court, asked for the revision of its competences. But the Member States didn’t accept to revise the Treaty of Rome and especially at the French request.  Later on, the Court underwent another important attack, this time coming from the British government. British euro-sceptics were highly interested to weaken the Court’s power, that’s why they forced their government to come up with proposals during the Maastricht Treaty negotiations. These proposals were about how to make the Court politically more accountable and to limit the costs of its decisions. The Member States rejected all the proposals of the British government, but they excluded the politically sensitive policy areas, such as Common Foreign and Security Policy and Justice and Home Affairs from the Court’s remit.  However, it is noteworthy that under the Treaty of Lisbon police and judicial cooperation in criminal matters are treated according to the same rules as those of the single market. As a result, those matters will be subject to the Court’s judicial review and any court or tribunal can ask it for a preliminary ruling. 
As it has been mentioned above, the Court manages to maintain its authority because it is very difficult for the Member States to reverse the Court’s decisions. The reversal requires at least the proposal of the Commission, the qualified majority voting of the Council and possibly a co-decision with the European Parliament. Since the Court’s decisions affect differently the Member States, they are not likely to enter into a coalition to change the unwanted decision. As Alter argues, it is easier for a Member State to implement the Court’s decision “by interpreting it narrowly or by buying off the people the decision affects"  , than try to reverse that decision. 
Another way to reverse the Court’s decisions is the treaty amendment, but it is even more complicated. As Pollack argues it is a “nuclear option - exceedingly effective, but difficult to use - and is therefore a relatively ineffective and noncredible means of member state control". 
In fact, in order to change the treaty, the unanimity of all 27 Member States, then the ratification in all of them are needed. As the interests of the Member States vary, and since all of them are veto holders, in reality it will be very hard to achieve unanimity and reverse the Court’s decisions by the treaty amendment. 
Finally, another reason explaining why the EU has such a strong Court of Justice relies on the fact that once appointed the judges are independent from their Member States, thus the latter have no way to influence the Court’s decisions through them. Taking into account the collegiate nature of the Court’s rulings, the Member States don’t have the possibility to find out how their judges voted in a particularly controversial or politically sensitive case.  They can’t even dismiss their judges, as according to the Treaty of Lisbon a judge may be deprived of his office by the unanimity of the judges and Advocates-General if “he no longer fulfils the requisite conditions or meets the obligations arising from his office". 
Our research allows us to conclude that the Court of Justice is nowadays an influential political and judicial institution. First, it is strong as it has managed to develop in the 1960s fundamental legal doctrines, namely those of direct effect and supremacy of the EU law. They are indeed crucial for the Court since they contributed to its further development and opened a road for gaining more power and authority in the future.
Secondly, the alliance with the national courts empowered the Court by enlarging significantly the scope of its authority.
Thirdly, the Court is emboldened by the fact that it is very hard for the Member States to reverse its decisions because of the lack of consensus among them. Moreover, we found out that the Member States are absolutely incapable to influence the Court’s decisions through their own judges.
And finally, it can be concluded that without a strong Court of Justice it is hard to imagine the process of the European integration and especially, the integration of the single market.
As Mancini mentioned “[...] no doubt as to the degree of activism the Court displayed in fostering the integration of Europe and forging a European identity. Judicial activism, however, is not necessarily a good thing".  As to the possible consequences of this “judicial activism"  of the Court of Justice, we can assume that they will become apparent only through time.