"We showed up to work the next day," the official said.
US President Barack Obama has sought to deflate public anxiety. "There's been a little bit of hysteria post-Brexit vote, as if somehow Nato's gone, the transatlantic alliance is dissolving, and every country is rushing off to its own corner," the president said in an interview with NPR that aired today. "That's not what's happening."
But security experts warn that much remains unknown, even for senior government officials, about what real-world consequences may result from Britain's departure from the EU in the relatively insulated world of military and security ties.
"We are writing a completely new chapter in Europe's history," said Heather Conley, a former US State Department official who is now a scholar at the Center for Strategic and International Studies in Washington. "The uncertainty absolutely is going to be an all-absorbing activity for Europe and the UK, and we don't know what that impact will be."
The vote, which stunned US officials at the Pentagon and other government agencies across Washington, is likely to dominate next month's Nato summit in Warsaw, potentially overshadowing discussion about ongoing conflicts in Ukraine and Afghanistan.
For decades, the US has conferred on Britain, its most important military ally, more special military privileges than any other country. The US government shares more intelligence with Britain than it does with other nations; Britain is now the only country to have lethal US drone technology; and British military personnel can regularly be seen in the halls of the Pentagon.
"[What] came out of World War II and then got us through the Cold War was a very close defence and security relationship, one that is intertwined in a lot of ways," the US official said, also speaking on the condition of anonymity to discuss bilateral defense ties.
Since the attacks on September11, 2001, the Pentagon has relied on Britain's much smaller but highly skilled military in operations across the globe. After 9/11, British troops joined their American peers in the Nato-led mission in Afghanistan, as they did in the US-led war in Iraq. Today, both the US and Britain have small numbers of elite forces in Libya.
Officials from both countries said the reason why the bilateral defence relationship will be relatively unchanged is that most of the two countries' military cooperation takes place on a bilateral basis or through Nato. There are no major agreements or any military arrangements that will need to be renegotiated.
Speaking after the vote, British Defence Secretary Michael Fallon said his country will not back down from its duty to combat international threats.
"This is not Britain turning its back on the world," he told Sky News. Today, Britain has more than 250 troops training and advising local forces in Iraq. According to British media reports, the government of Prime Minister David Cameron is considering augmenting its small military force in Afghanistan, in light of the Taliban's resurgence there.
The next UK government, after Cameron steps down this fall, is sure to share European concerns about an increasingly assertive Russia and will likely continue, for the near term at least, its participation in an air policing mission in the Baltic region and other activities that seek to respond to Russian President Vladimir Putin's military posture.
Fallon said Britain would announce a decision at the upcoming Warsaw summit to place troops closer to Europe's eastern edge as a response to Russian actions there.
Evelyn Farkas, who served as a senior Pentagon official for Russia, Ukraine and Eurasia, said the difficult task of extricating Britain from the EU has the potential to distract European leaders from the ongoing conflict in Ukraine but ultimately will not change the British position against Russia's expansionist moves.
"As much as Putin might be applauding this and pointing to the weaknesses and divisions within Europe . . . he's not going to be able to reap much reward from this directly," she said. "The Brits are just as much with us on the issue of Crimea and Ukraine . . . and this isn't going to change that."
Farkas said US military leaders will, however, need to hold Britain to earlier commitments, such as its decision to send British personnel to train Ukrainian troops. "DOD will have to make sure we message to the UK that we want them to help us on items that are priority," she said.
Officials acknowledge that the one area where the exit vote has the potential to drag on Britain's military clout is defence spending. In 2014, the Cameron government committed to spending 2 per cent of Britain's gross domestic product on defence, making the UK one of only five Nato member states that did so in 2015.
Now, with its currency plummeting, its future trade ties up in the air, and its political landscape in disarray, Britain could well slip into recession, potentially shrinking defence spending along with economic activity.
That might also impact Britain's ability to pursue expensive military technology. The British navy is preparing to launch an aircraft carrier that would eventually host US-designed F-35 jets.
It's too soon to tell how the post-exit realities for Britain will impact efforts to defend Europe from terrorist attacks like those in Paris and Brussels, or how it will shape officials' willingness to commit troops and resources to far-away conflicts.
"There has always been this strain within the UK, like in the US and other places too . . . wariness about doing things overseas," the US official said. "After the Brexit I would hope that the government and the people will see how important it is that they continue."
While last week's vote was an expression of many Britons' desire to put national interests first, American officials voiced cautious optimism that British leaders would uphold their tradition of employing military power, allowing the two countries' battlefield partnerships to continue.
