Not many people have a clear and consistent definition of what sovereignty actually is, or why countries can have it and no other actor can. However, in the arguments of eurosceptics, at least, it means a state being able to make its own decisions on whatever it wants, however it wants, without any outside interference. As a result of EU membership, some transfer of power from the nation states to the EU institutions occurs. This is why Eurosceptics argue that nations cannot be truly sovereign in the EU. However, that both ignores the idea of pooled sovereignty and also that member states are also sovereign in the traditional sense anyway. There is also the argument to be made about levels of governance below the member state. Is a state sovereign? A city? Why not?
All the EU member states are collectively sovereign through their membership of the European Union through a concept known as ‘pooled sovereignty’. Individually, European member states are weak, isolated, and can be dictated to by the world superpowers, having to follow their rules without having a say on them. Acting through the European Union, they have the ability to make their mark on world affairs and negotiate as an equal to those superpowers. As Donald Tusk stated on the occasion of the 60th anniversary of the Treaties of Rome, “Only a united Europe can be a sovereign Europe, in relation to the rest of the world.”
The more traditional idea of nation-state sovereignty also isn’t eroded by being in the EU. Member states actively chose to join, and can choose to leave at any point. Any power that was transferred to EU institutions as part of their membership of the European Union can be reclaimed, as the United Kingdom is demonstrating. Even the Department for Exiting the European Union’s (a UK government department) white paper admitted as much.
The reason Eurosceptics get worried about sovereignty is because theoretically an EU member state could be continuously outvoted and have to abide by EU laws they didn’t agree with. However, that ignores both the fact that most Council votes are adopted by consensus and aren’t contested, but also that similar things happen below the level of the nation state. Does Scotland always agree with the UK? Does Aberdeen always agree with Scotland? Does a ward in Aberdeen always agree with the city council? Of course they don’t always agree, but people argue about what level decision making should take place at.
Together, Europeans exert an influence on world affairs they wouldn’t otherwise be able to achieve individually, making EU member states collectively sovereign as a result of some pooling of power into the EU institutions from the member states. As well as that, member states can leave the Union at any time by activating Article 50, and can revoke the power granted to the EU, thereby demonstrating that member states can be, and are, sovereign with the European Union.