[NoboruWataya]

The supremacy of EU law is a pretty interesting one. It is a fundamental principle of the EU that EU law takes primacy/supremacy over national law (in areas where the EU has competence). How this actually works in practice can be a bit fuzzy, because the EU is certainly not going to send tanks into a member state's capital to enforce its laws.

As I understand it, the way this usually works is by national law explicitly endorsing EU law (usually at the level of the national constitution) and stating that in the event of any contradiction between EU law and domestic law, EU law will prevail. So EU law is "supreme" in practice, but that supremacy is granted/recognised under the domestic constitutional order.

In some countries, this recognition is limited, such that national courts will not permit EU law to override certain aspects of the national constitutional order. When that happens, there is really no easy solution.

An interesting recent example is https://www.politico.eu/article/brussels-closes-case-against... where the German court found that an ECB bond-buying programme was unconstitutional and in doing so refused to follow a prior decision of the ECJ.

Another consequence of this approach to supremacy is that significant changes to the EU treaties require a constitutional amendment in Ireland, which requires a referendum. To my knowledge Ireland is the only country to have such a binding legal requirement, with the effect that a number of amendments to the treaties have in the past been delayed or defeated by the Irish public voting against them.