LONDON – Boris Johnson succeeded Theresa May as Britain’s new prime minister on Tuesday and the incoming leader faces a bumpy ride amid pressure to get the nation’s stalled exit from the European Union – known as Brexit – over the line.  

Britain’s 77th prime minister will also have to deal with an escalating crisis with Iran. 

American-born Johnson, 55, who enjoys a good relationship with President Donald Trump, becomes Britain’s 14th prime minister to serve under the reign of Queen Elizabeth II. He will be the third leader from the United Kingdom’s Conservative Party, including David Cameron – who called 2016’s controversial referendum on EU membership – charged with making sure that Brexit takes place.

In a brief address, Johnson said he would “deliver Brexit” and “unite the country.”

Johnson won about two-thirds of eligible votes in a weeks-long contest decided by an internal party vote by approximately 160,000 Conservative Party members after May stepped down over her handling of Brexit. Britain elects a party, not a leader, meaning the ruling party can change its leader and still remain the government of the day. 

Johnson beat Jeremy Hunt, a close political ally of May’s. 

“Congratulations to Boris Johnson on becoming the new Prime Minister of the United Kingdom. He will be great!” Trump tweeted. 

While Johnson was the odds-on favorite to win, he will be quickly tested. 

He inherits a government that has repeatedly failed, three years after the EU vote, to find a practical solution to a seemingly intractable problem: While the British public narrowly opted to depart the 28-nation bloc, the majority of British lawmakers still don’t believe it’s in the nation’s economic or diplomatic interests. 

Amid an unraveling nuclear deal between world powers and Iran – precipitated by Washington’s withdrawal, a year ago, from the accord – Tehran has seized a British oil tanker in the Strait of Hormuz, an important waterway in the Persian Gulf.

Britain has found itself caught between the Trump administration’s “maximum pressure” campaign on Iran with its renewed sanctions, asset freezes, deployment of extra troops to the region and the designation of Iran’s Revolutionary Guards as a terrorist group, and its commitment to the nuclear deal.  

Back at home, many economists and political scientists believe Brexit could substantially damage Britain’s standing in the world and usher in dramatic changes on everything from its border security to food standards and human rights. It could also impact the status of Scotland, where political nationalists have vowed to try to exit the union of England, Scotland, Wales and Northern Ireland if Brexit is completed.  

“There are many people in the Conservative Party who want Brexit more than anything else on this Earth, and they are prepared to take not only a desperate act, but perhaps also an unconstitutional one,” said Timothy Bale, a professor of politics at Queen Mary, University of London, referring to Johnson’s refusal to rule out suspending Parliament in order to force through a so-called no-deal Brexit if it does not happen by October 31. Hunt had adopted a more cautious approach, ruling out hard deadlines.   

Meanwhile, EU leaders say they are not willing to renegotiate the terms of an exit deal already agreed upon with May, and which ultimately led to her ouster. Of particular concern to both parties: Northern Ireland’s border with EU member Ireland. 

Britain’s Parliament rejected May’s EU deal, largely because of a measure designed to ensure an open border for goods and services as enjoyed by all EU members. Johnson thinks the “backstop” keeps Britain too closely bound to EU rules. 

An invisible border is also crucial to the regional economy and underpins the peace process that ended decades of violence in Northern Ireland.

Johnson’s leadership appears to increase the chance of a “no-deal” Brexit, a scenario that would see decades of EU legislation that covers areas from aviation to trade effectively evaporate overnight. The Confederation of British Industry, a business lobby group, has warned a “no-deal” Brexit would lead to the largest decline in business investment in Britain since the 2009 financial crisis. Economic growth would be weaker.

Companies and members of the public have been stockpiling essential materials, foods and medicines, according to market research firms such as Blis. In Northern Ireland, officials fearing potential “no-deal”-related power shortages have drawn up plans to requisition electricity generators from the British Army in Afghanistan.

Polls suggest a majority of Britons oppose a no-deal Brexit and Britain’s Parliament has taken steps to ensure that lawmakers are not bypassed on any “no-deal” decision.

Johnson has vowed to bridge this impasse, partly by injecting new energy into Britain’s highest office, saying Tuesday he would help “bring a new spirit of can-do.”

“We are once again going to believe in ourselves, and like some slumbering giant we are going to rise and ping off the guy ropes of self doubt and negativity.”

Many Americans may not recognize Johnson’s name.

But they may know about his unruly mop of blond hair and gaffe-prone speeches, which have drawn comparisons to Trump, not least by Trump himself. 

“He’s a different kind of a guy, but they say I’m a different kind of a guy, too,” Trump said approvingly of Johnson last week. “We get along well.”

A former journalist, London mayor and foreign secretary, Johnson is never far from British tabloid headlines. He is known for such antics as getting stuck hanging on a zipwire over the Thames River or being caught on camera tripping a child during a soccer game. He has been criticized for his inflammatory use of language, calling Muslim women who wear the burqa – an enveloping outer garment – “letter boxes” and “bank robbers.” Johnson attended the most prestigious schools and colleges. He was born in New York, but gave up his U.S. passport amid a tax probe.

Hunt was a successful entrepreneur before going into politics. He replaced Johnson as Britain’s current foreign secretary. Before that, as health secretary, he presided over aggressive reforms to Britain’s beloved public health service that split public opinion. 

Iain Duncan Smith, a Conservative Party politician who once led the party, said that whoever occupies No. 10 Downing Street – the prime minister’s official office and residence – the “U.S. is simply the strongest ally that we have” and “making sure that the U.K. and the U.S. are as much as possible together on critical events is vital.”

Smith said that it was right that Britain’s ambassador to the U.S., Kim Darroch, resigned this month after leaked diplomatic cables he sent back to London described Trump’s White House as “inept” and “clumsy.” His comments led to a fierce response from Trump. During a live TV leadership debate with Hunt, Johnson refused to explicitly back Darroch, in contrast to Hunt who called Trump “disrespectful and wrong.”  

Outgoing leader May offered her “full support” to Johnson. 

She will officially tender her resignation to the queen on Wednesday. Johnson will take office later the same day, following his own audience with the monarch.