Britain’s decision to leave the European Union could make it more difficult to maintain transatlantic agreement on sanctions on Russia over Ukraine, and distract Britain and the E.U. from other pressing foreign policy issues as they disentangle their ties, analysts said Friday.

On the day after the historic vote known as Brexit, with financial markets plummeting worldwide, foreign policy and national security analysts were trying to predict the implications for the United States in its relations with the E.U and Britain, allies that separately and together have been key partners on a host of global issues, from the war in Syria to the nuclear deal with Iran.

Some adversaries immediately saw chinks in Europe’s armor as a result of the vote. Politicians in Moscow predicted it would sap the E.U. of a strong member and ultimately hasten the lifting of sanctions with damaging effects for the United States.

“In my opinion, the most important long-term consequence of all this is that the exit will take Europe away from the anglo-saxons, meaning from the USA,” said the Kremlin’s small-business ombudsman, Boris Titov, in a Facebook post. “It’s not the independence of Britain from Europe, but the independence of Europe from the USA.”

Michael McFaul, a former U.S. ambassador to Russia, tweeted as much, writing that Russian President Vladi­mir “Putin benefits from a weaker Europe. UK vote makes EU weaker. It’s just that simple.”

When President Obama visited London in April, he said Britain’s membership in the E.U. “magnifies” its influence and warned they would have to go in “the back of the queue” for future trade deals if they left. He sounded more conciliatory on Friday, saying the “special relationship” between the two countries would endure.

But analysts said the British vote could signal a more inward-looking Europe as it copes with the decision.

“One of the strongest cards we have to play against Putin is transatlantic unity,” said Julianne Smith, a national security analyst with the Center for a New American Security. “This creates two years of navel gazing and internal debate about where do we go from here and Britain’s place in Europe. We lose our ability to stand together.”

The loss of a British voice within the E.U. means Washington will have one less like-minded friend in diplomatic issues involving Europeans.

“It’s still an important partner, one of the most militarily capable and diplomatically accomplished partners there is,” said Karen Donfried, president of the German Marshall Fund of the United States. “To not have the E.U. benefit from the role Britain plays is a net negative.”

But some analysts say the concerns are being overblown. Britain’s military will continue to operate within NATO, which is separate from the E.U.

Britain is providing troops and equipment to coalitions that are waging military operations in Iraq, Yemen and Syria. Michael O’Hanlon, a national security analyst with the Brookings Institution, said that while Britain’s contribution is larger than its size would warrant, it is still no more than 10 to 15 percent of what the United States commits.

“The stakes are important, but they’re not astronomically big to begin with,” he said.

“It may have repercussions down the road,” he said of Britain’s breakup from the E.U. “Maybe its military will become a little smaller. But we need to take a chill pill on worries about the downsides of withdrawal.”

Indeed, some argue that Britain’s exit from the E.U. could free London to act more forcefully and, for instance, impose harsher sanctions against Russia.

“A sovereign Britain, one able to act completely independently of the supranational European Union, will be a more powerful force on the international stage than it is now,” said Nile Gardiner, a onetime aide to former Prime Minister Margaret Thatcher who now heads the Heritage Foundation’s Center for Freedom that is named after her. “I think that far from welcoming Brexit, Putin fears it.”

The biggest unknown is whether the British move will have a cascading effect.

“If three or four other countries do this as a copycat, then we have a more interesting problem,” said Barry Pavel, director of the Atlantic Council’s Brent Scowcroft Center on International Security. “They’re opting out of an economic union, and I don’t think it’s good, but it’s not as catastrophic as people are playing out — yet. We have to wait and see what happens.”

Anti-E.U. parties in several European countries have said they plan to agitate for a referendum as Britain did. Marian L. Tupy, who analyzes the effects of globalization at the Cato Institute’s Center on Global Liberty and Prosperity, called the British vote “the beginning of the end for the European Union.”

“With every electoral cycle, pro-European parties are losing support and parties the E.U. calls dismissively ‘populist’ are increasing in popularity,” he said. “It was only a matter of time before an E.U. country had either elected an anti-European government or held a referendum on it. The British were the first.”

Tupy, who supports the breakaway as an expression of freedom, considers the security aspects negligible. The United States shares its intelligence more judiciously with other countries on the European continent than it does with Britain, he said.

But Smith of the Center for a New American Security said intelligence sharing and cooperation inevitably will be weakened in a divided Europe.

“Whether it’s counterterrorism or a resurgent Russia or challenges in the Middle East, our ability to put forward common strategies is in question in a very distressed Europe,” she said. “This is a crushing blow.”