At its simplest level, sovereignty is established by national legislative supremacy. That is to say, that a national Congress or Parliament makes the law and then national judges interpret and apply the law.

But that sovereignty is exactly what the European Union is demanding Britain qualify as part of its Brexit negotiations.

Enter Michael Barnier, the EU's chief Brexit negotiator. A former French conservative politician and well-regarded bureaucrat, Barnier is responsible for representing the EU's interests in Brexit. That means he's likely a high-priority collection target for GCHQ (Britain's NSA), and that he will receive honored status in visits to the U.K. Such is the intersection of statecraft and the pageantry of diplomacy.

But it also makes Barnier a man who must be listened to by all those concerned about the shape and form of Brexit.

On Wednesday, Barnier demanded that the European Court of Justice retain its authority over the interests of EU citizens in post-Brexit Britain. According to Barnier, the European Court of Justice is the "ultimate guarantor" of European rights. Without its continued control over the treatment of EU citizens in Britain, Barnier believes those citizens may suffer unfair treatment in U.K. courts.

Barnier might pretend this is about protecting innocent people from injustice, but it's nothing of the sort.

For a start, the English courts of appeal (the High Court, Court of Appeal, and Supreme Court) have shown immense deference to individual claims against government overreach. Were a future British government to unjustly restrict EU residents from exercising their rights, the courts would protect them.

Of course, Barnier knows this. So what's really going on here? Two things.

First, and most important, Barnier is advancing the EU's broader agenda of retaining control over British governance.

At present, the European Court of Justice is the supreme legal authority over Britain. Unfortunately, as I've explained, "the court substitutes precedent-based jurisprudence for federal expansionism. And voters see the court for what it is: a political mechanism, shielded from democratic authority, that systemically degrades national democracies." In practice, the European Court of Justice is a hyper-political entity that exists to construct law around the pursuit of a United States of Europe. That's not its stated mission, but it is what the court does.

Second, Barnier wants to ensure that EU citizens living in Britain are able to bring their families to join them. As the BBC notes, this freedom of movement principle is one that the British government is reluctant to accept. Doing so would restrict the government's ability to reduce immigration. Yet by establishing the European Court of Justice as supreme authority on this matter, Barnier can ensure that the EU decides whether EU citizens can bring their families to the U.K.

Ultimately, these two considerations explain why the UK government cannot yield to Barnier's demands.

Yes, on other issues such as a $60-90 billion payment to settle its program commitments with the EU authorities, Britain must show flexibility. That said, the principle of sovereignty is too important, indeed too defining, to be negotiated away. The British government can offer commitments on EU citizen rights post-Brexit. It must not, however, allow EU judicial review of British law.