For centuries, this modest little island in the North Sea has punched well above its weight on the international stage: It built a global empire, beat back the Nazi tide and stood shoulder to shoulder with the United States during a decades-long standoff with the Soviets.

But now that Britain has stunned the world with its decision to exit the European Union, experts say it will be focused inward for the foreseeable future.

“I don’t think there will be the capacity or the infrastructure to look outward in the next five years,” said Ian Kearns, director of the London-based European Leadership Network. “With all our diplomatic resources focused on extracting concessions from the E.U., we won’t be in anything other than reactive mode on other issues.”

That reality could bring a significantly diminished role on the great challenges facing the West, including Russia, the Islamic State, refugees and climate change.

In a stunning victory for the "Leave" campaign, Britain has voted to exit the European Union. Here's what happens next. (Jason Aldag,Adam Taylor/The Washington Post)

For Washington, Britain’s distraction will be acutely felt. Britain has long been the United States’ closest ally, one that broadly shares American interests and values, and has always formed a crucial bridge across the Atlantic.

The United States looked to Britain when it needed to influence European decision-making. The E.U. turned to Britain when it hoped to influence the United States.

Now, the loss of Britain’s voice in efforts to present a united European and American front on issues such as sanctions against Russia is particularly worrisome to U.S. officials, said Philip Gordon, a former assistant secretary of state for European affairs in the Obama administration.

“That voice will no longer be there when withdrawal is complete,” Gordon said. Instead, Britain will be preoccupied with its “great domestic convulsion.”

Thursday’s referendum was the most dramatic blow to Britain’s role in the world, but even before that, a series of events caused Washington to fret that its closest ally was pulling back from its customarily active global role.

Parliament’s 2013 vote against airstrikes in Syria, Britain’s absence from Ukrainian peace talks and its delay in joining the air campaign against the Islamic State all contributed to a feeling that Britain was becoming a less-reliable partner in international affairs.

Meet the European leaders hoping to cause the next Brexit

A victory for the “remain” camp in the referendum vote was supposed to be a turning point: With Prime Minister David Cameron having put the country’s two great existential dilemmas behind it – Scottish secession and E.U. membership – he would have a free hand to reassert Britain’s role as a global power.

But Cameron’s gamble of calling a referendum went badly wrong, and now Great Britain faces the very real prospect of being transformed into Little England.

Scotland is once again pressing the case for secession. Pro-Brexit voters are demanding that the country shut the door to large numbers of immigrants. And the country’s political leadership and diplomatic corps are likely to spend years conducting divorce proceedings with soon-to-be former partners in the E.U.

“It’s taken a step to withdraw from the world,” said Tom Donilon, former national security adviser to President Obama. “It will go from being an important participant in the European
decision-making mechanism to being a party on the other side of the negotiation table.”

Donilon said that for as long as most people can remember, Britain has been the first call made by a U.S. president when there is a challenge in the world.

But Thursday’s vote means calls between the White House and 10 Downing Street could matter less.

“They’ll remain an important player in NATO and an ally of the United States. But they’ve diminished their leverage in Europe,” he said. “Their membership in the European Union amplifies British influence. They’ve pulled back from that. I don’t think that is good for Britain. I don’t think it’s good for the United States.”

British politicians who campaigned for Brexit dispute that logic, arguing that a Britain freed from the shackles of the E.U. will be better able to assert its interests around the globe.

Britain’s E.U. membership, they have said, diluted the country’s voice rather than amplified it, and actually inhibited its ability to develop its own relations with great powers like the United States, as well as rising ones like China and India.

Brexit campaigner and former London mayor Boris Johnson said Friday, after the votes had been counted, that Britain’s influence will be undimmed, even in Europe.

“I want to reassure everyone Britain will continue to be a great European power, leading discussions on defense and foreign policy and the work that goes on to make our world safer,” Johnson said. “But there is simply no need in the 21st century to be part of a federal government in Brussels that is imitated nowhere else on Earth.”

But foreign policy experts on both sides of the Atlantic dispute that logic, at least in the short and medium term.

The task of disentangling Britain from Europe will be gargantuan, given the extraordinary links across the English Channel in commerce, security and dozens of other areas. Britain will not only need to negotiate its way out of the 28-member bloc, it also must ink new trade deals and other agreements to replace the E.U. ones it has relied on to do business with the rest of the world.

The process probably will take up the rest of this decade and could reach well into the next one.

Britain also faces a threat to its very existence: Leaders in pro-E.U. Scotland said Friday that they will push for a new independence referendum. On Saturday, First Minister Nicola Sturgeon went further and said she would effectively circumvent London’s authority over foreign affairs by reaching out to Brussels directly to discuss “possible options to protect Scotland’s place in the E.U.”

A Scottish departure would even further limit London’s international clout, not least because Scotland is home to Britain’s nuclear arsenal — raising thorny questions of whether London would relocate the program or abandon it altogether in the event of a Scottish vote to leave.

Britain could also be struggling economically in the years to come. On Friday, the British pound plummeted and markets tumbled worldwide. Analysts say that’s just a taste of the pain to come for Britain, with a downgraded credit rating, job losses, higher interest rates and a fall back into recession all potentially on the horizon.

In that environment, the prime minister — whoever that may be following Cameron’s resignation — will be hard-pressed to maintain the country’s current spending on defense, diplomacy and international aid.

And if Britain thinks it will at least be free of having to worry about what’s happening in Brussels, it’s mistaken, said Robin Niblett, director of the London-based think tank Chatham House.

The country will still have to coordinate closely with its erstwhile E.U. partners, Niblett said, because “Britain is so tightly interlinked with Europe in terms of the problems we share — Russia, ISIS, refugees.”

The only difference, he said, is that coordination will be more difficult and time-consuming because Britain will be “on the outside rather than having a seat at the table.”

“The irony,” Niblett said, “is that we’re going to spend more time focused on Europe over the next 10 years, not less.”