One year after she triggered Article 50, British Prime Minister Theresa May has every reason to feel satisfied. She set in motion the two-year negotiation over Britain’s exit from the European Union without knowing how the process would work and without a clear idea of what she wanted out of it.
Many of her initial assumptions turned out to be wrong: She believed that the two sides immediately would start negotiating a new trade deal, she thought the EU’s need for British cash to plug a hole in the EU budget would buy the...