As the two sides gear up for the fraught negotiations about to ensue, the unreality is this: The EU seems unable to grasp the nature of Brexit; the U.K. seems unable to grasp the price it has paid to get it. The consequences could be a far more profound rupture than either wants or is necessary—and, for London, a far more difficult relationship with Northern Ireland (let alone Scotland).

To understand why all this is happening, going back to basics is important. The EU works as a collection of sovereign states that agree to give up direct national control over areas of daily life, including food standards and fishing rights, and international trade, immigration, and social standards. They do this, pooling national control into a body that sits above them, because they believe the benefits are worth it, whether in terms of prosperity, peace, or regional influence. Many Europeans, including Brits, also believe in the project as an expression of identity; they are European and not just French or Dutch, Irish or Greek.

Brexit is a rejection of this bargain. In seeking to “take back control” of Britain’s money, laws, and borders, London is exercising its right to leave the higher jurisdiction of EU law that keeps the organization together, accepting that, as a result, it cannot enjoy the benefits of membership: ease of trade and travel and, fundamentally, influence over the laws made in Brussels.

What has complicated the whole endeavor is that the U.K. is not a normal nation-state, but itself a union of nations. One of those—Northern Ireland—cannot even agree on whether it is a nation or not, and sits not on the island of Britain, but on the island of Ireland, sharing a land border with the Republic of Ireland, a separate country and EU member state, to the south.

If it weren’t for Northern Ireland, as Angela Merkel is alleged to have told Boris Johnson, Brexit would be more straightforward. But Northern Ireland does exist, and will remain a sovereign part of the U.K.—the United Kingdom of Great Britain and Northern Ireland—until a majority of its population chooses otherwise. Yet almost half of its citizens consider themselves Irish, not British, and would resent any post-Brexit disruption to their lives crisscrossing the border with the Republic. To maintain the status quo, London, Dublin, and Brussels agreed to keep the border open as before.

Yet the only way to keep the border open was for EU law to apply north and south of it. In other words, EU law would have to continue to apply in a part of the U.K., even after the U.K. had left the EU. London was then faced with a choice: Should the whole of the country accept EU law, or should an internal border be erected? Theresa May chose the former and lost her job, Boris Johnson chose the latter and won a landslide general-election victory.