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.

Johnson is undaunted, however. After receiving the queen’s assent to become prime minister, he warned that “The doubters, the doomsters, the gloomsters … are going to get it wrong, again.” He insisted that “in 99 days’ time we will have cracked it” and the U.K. will be able to exit the EU with “a new deal, a better deal.” It would seem that all is needed is a new attitude. Johnson blamed “three years of unfounded self-doubt” and urged his fellow citizens “to recover our natural and historic role as an enterprising, outward-looking and truly global Britain, generous in temper and engaged with the world.” To assist him in that effort, he has replaced all members of May’s Cabinet with loyalists and Brexit true believers.

It will take more than “the pluck and nerve and ambition of this country” to solve the problems Johnson faces. He was elected head of the Conservative Party, polling twice as many votes as his rival, Foreign Secretary Jeremy Hunt. But that ballot involved party members only, so 139,000 people out of a population of 66 million elected him, and surveys show 58 percent of Britons have a negative opinion of him. His first objective, therefore, is winning over the majority of people who dislike him.

It is not clear what new ideas he has to secure a better agreement with the EU, whose officials have made it clear that they will not be renegotiating their deal with May. The EU’s chief negotiator deemed “unacceptable” terms Johnson articulated in his first speech to Parliament as prime minister. His leverage seems to be the threat of withholding the $50 billion that May promised as part of her agreement. Those who know or have observed Johnson fear that he does not in fact have much more than that. He is famous for being unprincipled — he famously wrote two editorials on the eve of the Brexit vote, one advocating for, the other against — as well as ill-prepared, if not indifferent to details. Individuals who have worked closely with him confess that they do not know what policies he supports or what he believes in.

Johnson also inherits a standoff with Iran, which has seized a British tanker in retaliation for the U.K.’s detention of an Iranian ship in the Mediterranean. That is part of the larger problem of salvaging the multilateral nuclear deal with Iran from which the United States has withdrawn. U.S. President Donald Trump considers Johnson to be a kindred spirit, and expects him to join the U.S. “maximum pressure” campaign to force Tehran to renegotiate the nuclear deal. Johnson cannot afford to antagonize Trump because he also hopes to conclude a trade deal with the U.S. as part of his Brexit solution.

Japanese businesses view “no deal” with trepidation. In his congratulatory message to Johnson, Prime Minister Shinzo Abe repeated warnings about the impact of Brexit without an agreement. Japan has other reasons to worry about Johnson, however. He claims to be very “pro-China,” which could complicate Tokyo’s attempts to shape Beijing’s behavior. Johnson’s support for the “Belt and Road” initiative will complicate efforts to reign in the excesses of that project. Given the U.K.’s precarious status after Brexit — it will need new deals with every major trade partner, including Japan — it is unlikely that London will push hard against Beijing on other matters.

That is the future, however. First, Brexit looms. A poorly navigated Brexit threatens not only the British economy but the existence of the U.K. itself: Scotland rejected Brexit and a debacle could force it to reconsider its membership in the U.K. If that follows, Johnson will go down in history for all the wrong reasons.