THE GOOD news from Britian’s election is that the political impasse that has paralyzed the country since it voted to leave the European Union in 2016, and made it a weaker partner for the United States, at last is over. Prime Minister Boris Johnson won a resounding mandate, capturing the largest parliamentary majority for the Conservative Party since Margaret Thatcher in 1987. For the first time in a decade, Britain will have a strong leader.

The wrinkle is that, apart from formally ending the country’s E.U. membership by Jan. 31, it’s not clear how Mr. Johnson might use that mandate. Having governed London as a centrist mayor, he has since shifted toward a populist nationalism that earned him the admiration of President Trump. That helped him to win over working-class voters in central and northern England and capture dozens of seats that for decades were strongholds of the Labour Party.

Mr. Johnson will surely seek to “get Brexit done,” as his campaign relentlessly promised. But this momentous change in the country’s economic and political alignment could be more or less sweeping — and damaging to its economy — depending on what happens after Jan. 31. Britain has yet to negotiate its future relationship with the E.U., determining how much access it will have to its markets, how much it will depart from European regulation, and how free it will be to negotiate separate trade deals with the United States and other countries.

Mr. Johnson, who helped lead the 2016 referendum campaign for Brexit, has seemed inclined toward a sharper break, and he has promised not to extend a transition period beyond the end of next year, which could cause Britain to crash out of the E.U. market without a deal. But the big Conservative majority may give a leader notorious for his opportunism leeway to shift back toward a more centrist course. That would be wise, given the dangerous political polarization that has overtaken Britain since 2016 — and the risk that the union may split apart.

The most consequential result of the election, apart from Mr. Johnson’s triumph, may have been the landslide victory in Scotland of the Scottish National Party, which opposes Brexit and favors independence. On Friday, its leaders were already calling for a new referendum on independence, something Mr. Johnson opposes but may find difficult to resist. The allegiance of Northern Ireland and even Wales to the United Kingdom may also crumble if Brexit is poorly managed.

The Labour Party may or may not be able to contribute to political healing. The disastrous loss it suffered under far-left leader Jeremy Corbyn was its worst since 1935. That offers comfort to people in and outside Britain offended by Mr. Corbyn’s statist socialism, his sympathy for extremist movements from Latin America to the Middle East, and his tolerance of anti-Semitism. But it’s not yet clear that Labour will respond to its loss by moving back toward the center; though his career may be over, Mr. Corbyn’s followers retain a strong hold on the party.

As in many Western democracies, a vacuum has been growing in Britain’s political center. Mr. Johnson’s big win, big personality and political flexibility offer him the chance to fill it. To do so, he will need to embrace the tolerant pragmatism that once endeared him to London — not the divisiveness that charmed Mr. Trump.

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