As U.K. Prime Minister Theresa May starts the clock on Britain's departure from the European Union, there's no real question that the divorce is going to hurt both parties — and the United Kingdom, to be sure, much more than the EU. But this isn't to say there's no upside. The right kind of deal will recognize these opportunities and try to make the most of them.

This whole misadventure began with historic errors of judgment — former British Prime Minister David Cameron's decision to call a referendum on U.K. membership, and Europe's decision to deny him significant concessions in the talks that followed. Further miscalculations over the coming months are likely and could easily make a bad situation worse. But a friendly parting is still possible, and in both sides' best interests.

The main argument against this view is that the EU should rationally want to punish the U.K. for its choice, to deter other countries from following its lead. Too much is made of this.