Three weeks after a majority of Britons voted in a referendum to leave the European Union, the British political landscape looks entirely different.

David Cameron, who called the referendum, is no more the Prime Minister. Boris Johnson, an exit campaigner who was widely expected to replace Mr. Cameron, backed off even before the contest for the new Prime Minister began. Michael Gove, another Brexiteer who entered the race, was rejected by Conservative MPs. Theresa May rose from this post-referendum chaos to become the second woman Prime Minister of the U.K. A seasoned politician with administrative experience, Ms. May’s style of working and policy preferences often invoke comparisons with Margaret Thatcher and Angela Merkel. As Home Secretary for six years, she oversaw Britain’s security services, borders and police forces. Despite her hard-line positions on immigration – at the Home Office she supported a net immigration cap – she chose to back the Remain camp, like Mr. Cameron, during the referendum campaign. This pragmatic Euroscepticism may have helped her win over both the doves and hawks within the Conservative Party.

That the U.K. has put an end to political uncertainty quicker than expected is good news for both the country and Europe. But the challenges Ms. May faces are unprecedented. The Conservative Party is divided. Legislators and other party leaders may endorse her for now, but going forward she could find it tough to maintain the equilibrium between the centrists and right-wing conservatives. Mr. Cameron’s decision to call the referendum to appease the right-wingers shows how unstable that equilibrium can be. Secondly, the Tories were re-elected last year under Mr. Cameron’s leadership on promises of fixing the country’s economic worries. The Brexit vote has already done damage to the fragile economic recovery. Ms. May’s immediate task would be to restore investor confidence. Thirdly, there is an alarming rise of xenophobia in the U.K. which threatens its social cohesion, which no ruler can ignore. A yet larger challenge for Ms. May would be dealing with the Brexit referendum outcome. Mr. Cameron had promised to trigger Article 50 of the Lisbon treaty if there was a Leave vote. In the event, he did not. None of the Brexiteers managed to succeed him. Ms. May, herself a Remain supporter, faces a difficult situation. If she doesn’t begin the process of taking the U.K. out of the EU, she faces the wrath of Brexiteers within and outside her party. If she invokes Article 50, it could have immediate repercussions for the economy and London’s ties with Scotland. This is a tall order that even Ms. May’s idol, Margaret Thatcher, would have struggled with.