Brexit roiled global equity markets and geopolitics, and the messy succession process to replace Prime Minister David Cameron, who opposed the United Kingdom's leaving the European Union, amplified the uncertainty. So it is a reassuring sign that Theresa May, a leader widely described as steady and steely, has become Britain's next prime minister.

Like Cameron, May opposed Brexit (albeit quietly). But she wisely will act on voters' will. "Brexit means Brexit, and we are going to make a success of it," May said on Monday.

That can-do approach stands in stark contrast to Boris Johnson, who bailed on being a prime minister candidate after appearing to be completely unprepared to carry out a result he championed. (He'll have a chance at redemption as Britain's new foreign secretary.) Similar fecklessness doomed the candidacies of Michael Gove and Andrea Leadsom. And although the United Kingdom Independence Party's Nigel Farage wasn't a candidate, he also slinked off after Brexit's surprise victory.

May, conversely, looked like a leader, and her reputation as a well-­prepared, no-nonsense negotiator will serve her country well as she bargains terms of Britain's exit. She's smart not to trigger Article 50, the official separation process, until she's confident in her country's position. Even then Brexit will create economic disruption.

As important, May will face justifiable impatience from E.U. leaders who decry the uncertainty a drawn-out divorce would bring. But European leaders must at the same time work with May in responding to the continent's multiple crises. Conflict and poverty will continue to spur the Mediterranean migration crisis; Vladimir Putin may be even further emboldened to test Western resolve in his bid to weaken NATO and erode sanctions imposed on Russia for annexing Crimea and destabilizing Eastern Ukraine. And a persistently sclerotic European economy threatens to leave an entire generation underemployed and fuel even more unpredictable (maybe even extreme) politics.

Britain may be leaving the European Union, but it can't leave Europe; continental leaders should consider May a force for good.

Likewise, President Obama and his successor should make every effort to fortify the U.S.-U.K. "special relationship." This cohesion is essential for global leadership, especially given the English-speaking giants' status as two of just five permanent members of the United Nations Security Council. It's crucial that an E.U. crisis not become a NATO rift, since that vital alliance is already straining from inconsistent contributions from member states.

Britain's second female prime minister has led to comparisons to May's fellow Conservative Margaret Thatcher. But German Chancellor Angela Merkel might bear a stronger resemblance, Fran Burwell, vice president for European Union and Special Initiatives at the Atlantic Council, told an editorial writer.

"Theresa May is very well-suited temperamentally, in terms of expertise, in terms of experience, to be the prime minister during this key moment in British politics and history," Burwell said. But "she will not be someone who will be blindly following the Americans — she is always going to be thinking about British interests." And yet, Burwell concluded, Britain's course matters. "They're not huge, but they have world reach."

World reach, but likely distracted by Brexit's economic, political and diplomatic fallout. Allies must work with Britain's new leadership to keep the upheaval from intensifying.