LONDON — Boris Johnson, the brash standard-bearer for a British exit from the European Union, won the contest to become the next prime minister on Tuesday, at a critical moment in his country’s history and with less political clout than just about any of its leaders since the Second World War.
His Conservative Party holds only a slim working majority in Parliament. But he has nonetheless promised to carry out Britain’s labyrinthine exit from the European Union by Oct. 31 — a challenge that confounded his predecessor, Prime Minister Theresa May, for the three years she held office.
He will also enter 10 Downing Street at a moment when the country is confronting a crisis with Iran over its seizure last week of a British-flagged oil tanker, threatening to draw Britain into a larger showdown between Tehran and Washington.
And the new prime minister inheriting these challenges is arguably the most improvisational and least predictable politician in recent British history.
Mr. Johnson seemed to acknowledge some incongruity between the man and the moment.
“I know that there will be people around the place who will question the wisdom of your decision,” he said Tuesday after the results were announced at a Conservative Party meeting, “and there may even be some people here who still wonder quite what they have done.”
Mr. Johnson has a long track record of statements about Iran, Brexit and other subjects, but there is no consensus on how he might actually act as prime minister.
“That is what concerns me: none of us really know what Boris stands for,” said Michael Stephens, a scholar at the Royal United Services Institute who has worked under Mr. Johnson in the Foreign Office.
Even after recent campaign debates, “I still don’t know what he stands for,” Mr. Stephens added.
Rarely has a new prime minister faced so many disparate questions of such urgency on the first day in office, said Peter Ricketts, a former British national security adviser.
“This is pretty unprecedented, in the level of turmoil in British politics, the depth of divisions within parties and across parties. And it’s a hot national security crisis,” Mr. Ricketts said. “An inconvenient time to be changing over.”
The Conservative Party announced Tuesday that Mr. Johnson had won 66 percent of the postal vote to become the new party leader, defeating Foreign Minister Jeremy Hunt in the runoff. He is expected to visit Buckingham Palace on Wednesday for Queen Elizabeth II to formally assent to the transition.
“We’re going to get Brexit done on Oct. 31, we’re going to take advantage of all the opportunities that it will bring in a new spirit of can-do, and we’re once again going to believe in ourselves,” Mr. Johnson vowed anew on Tuesday. “Like some slumbering giant, we’re going to rise and ping off the guy-ropes of doubt and negativity.”
Mr. Johnson, 55, is a former journalist and the author of a Winston Churchill biography whose ambition as a child was to become “world king.” His singular brand of bluster, at once upper crust and irreverent, carried him to two terms as mayor of London. Then his pro-Brexit leadership propelled him to become foreign secretary under Ms. May, and now her successor.
[Read about Boris Johnson’s record as mayor of London.]
President Trump tweeted congratulations. “He will be great!” the president added.
Brexit will define Mr. Johnson’s legacy as well as Britain’s place in the world, and his promise to pull it off by Oct. 31 “do or die” has already met deep skepticism within his party.
His plans “will collide with reality,” Amber Rudd, the work and pensions secretary, had predicted.
The debate over Brexit has prompted renewed talk about possible Scottish independence and a united Ireland, raising questions about the durability of the United Kingdom itself.
Nicola Sturgeon, Scotland’s first minister, congratulated Mr. Johnson on twitter but added: “It would be hypocritical not to be frank about the profound concerns I have at the prospect of his premiership.”
For an orderly exit that minimizes economic disruption, Mr. Johnson must convince both the British Parliament and the European Union to agree on an exit deal, a task his predecessor, Ms. May, found impossible.
[Here’s what to know about the possibility of a no-deal Brexit under Boris Johnson.]
Rob Ford, a professor of politics at the University of Manchester, noted that Mr. Johnson’s personal style was “the polar opposite” of his predecessor. Ms. May “knows what she wants most of the time, but she doesn’t tell anybody,” he said.
“He doesn’t seem to have any fundamental commitments or attachments to anything, and seldom knows the details of policy, let alone what he wants to do,” Ford said.
If the two sides cannot agree, Mr. Johnson might seek to exit the union without any deal at all, a step that could mean suspending the Parliament, enduring a constitutional crisis and plunging the economy into chaos.
Mr. Johnson has declined to rule it out, perhaps hoping to use the threat as leverage in talks with E.U. leaders. But the costs of a “no deal” Brexit would be four times as large for the British as for the rest of the European Union collectively, one recent report found. The European Union receives about 13 percent of Britain’s economic output, while exports from the E.U. to the U.K. account for 2.5 percent of the bloc’s output.
In an interview, Tony Blair, a former Labour Party prime minister, called a “no deal” Brexit a huge risk for Mr. Johnson as well as for Britain.
If blocked by lawmakers, Mr. Blair said, a failed attempt at a “no deal” exit could force a general election or possibly a second referendum.
Mr. Johnson “will face the facts and decide that if you try to engineer no-deal without Parliament — against Parliament’s wishes — and without public endorsement, you better hope it works perfectly,” Mr. Blair said. “Because if it doesn’t, you’re going to be in all sorts of difficulty for the rest of your time in politics.”
Iran, though, will be Mr. Johnson’s most immediate problem.
The British flagged-tanker Stena Impero, seized by the Islamic Revolutionary Guards Corps on Friday as it sailed near the Strait of Hormuz, remains captive in the Iranian port of Bandar Abbas. With a steady stream of images and videos released by Tehran to hold the public’s attention, the British Parliament and news media have turned to debating how the government failed to anticipate the danger, why the royal navy could not protect the ship, and how to rescue the vessel.
At the same time, Iran is locked in an escalating confrontation with the United States as well. That dispute centers on President Trump’s withdrawal last year from a 2015 accord between Iran and the world powers that promised Iran sanctions relief in exchange for suspending and dismantling much of its nuclear program, which the United States and Britain feared could lead to a nuclear weapon.
The Trump administration this spring has stepped up a “maximum pressure” campaign of economic sanctions to force Iran to submit to a new and more restrictive agreement, and Iran has struck back by beginning to restart its nuclear program and flexing its muscles in the Persian Gulf — where it seized the tanker.
While demanding the release of the tanker from Iran the British government has so far also sought to push back against the United States in its escalating confrontation with Iran, and the walking of that fine line will now be up to Mr. Johnson.
Mr. Johnson has visited Tehran as foreign minister, and last year he made an emergency trip to Washington in an unsuccessful effort to press the Trump administration to stay in the deal.
Mr. Stephens, of the Royal United Services Institute, said Mr. Johnson showed a solid command of the details and knew how to stay on message.
“But then there is Boris the Showman,” he said, “and it just takes Showman Boris to do something stupid and then we are all in trouble.”
The tanker crisis, said Ellie Geranmayeh, a scholar at the European Council on Foreign Relations, “is a baptism by fire in the first week on the job” for Mr. Johnson, and although he has said he wants to avoid confrontation, “Boris is unpredictable. He could completely go the other way.”
Still, Frans Timmermans, the European Commission vice president, said Friday that she was ready for him. “The world’s politics is rife of colorful people these days,” she said, “so if you can’t deal with them, there’s not much you can do.