Alexander Boris de Pfeffel Johnson, better known to the world as Boris, has become prime minister of the United Kingdom. While that has been his life's ambition — as a child, Johnson said he wanted to be "world king" — he may be accepting a poisoned chalice. He takes office with few, if any, fixed beliefs, a reputation for being as disheveled in his policy as in his appearance, and facing a set of crises as acute as any with which his predecessors have had to deal.

Johnson's biggest challenge is Brexit, a cause for which he has been a leading campaigner. While Parliament has been unable to coalesce behind the terms in which the U.K. is set to exit the European Union on Oct. 31. Brussels has shown no inclination to renegotiate an agreement reached with his predecessor, Theresa May, and unhappy Brexiters have insisted that they will leave regardless. Johnson is with them, having pledged that Britain would leave the EU in October "no ifs or buts."

"No-deal" Brexit would be an economic catastrophe. Mark Carney, governor of the Bank of England, warned that it would deliver a "major economic shock" to Britain. The U.K. Office for Budget Responsibility estimated that it would cost the country £30 billion in public finances, while the U.K. Trade Policy Observatory put the cost of compensating British businesses at £22 billion. Nor is the danger to Britain alone: The International Monetary Fund identified a no-deal Brexit as one of the chief threats to the world economy.