U.S. Secretary of State John Kerry and U.S. Ambassador to NATO Douglas Lute arrive at the NATO headquarters in Brussels, Monday, June 27, 2016. Reuters/Eric Vidal

As world leaders meet to discuss Monday the fallout from Britain’s vote to leave the European Union, American officials stressed that the U.S. would maintain its special relationship with the U.K. But as both President Barack Obama and British Prime Minister David Cameron prepare to leave office and Germany takes a key role guiding the next phrases of the Brexit process, that relationship will depend greatly on who steps into both men’s shoes.

Nicholas Burns, a former State Department official and adviser to presidential candidate Hillary Clinton, told the Financial Times in an article published Sunday that when a question needs to be settled in Europe, American officials are increasingly looking to Berlin and not London, a close ally for many decades.

“Britain introduced us to the EU and introduced the EU to us, playing a pragmatic role. But much of that will be gone,” Burns said.

The U.S.-U.K. relationship has entered a moment of strain and transition, P.J. Crowley, a former U.S. assistant secretary of state, told BBC News Sunday.

"The unanswerable question in Washington is what challenges lurk as they shift from the region that is to the one that will be. ... What we don't know is whether they will be more effective," Crowley said.

Political instability and uncertainty lie ahead in the U.K. for two to three years as negotiations begin over Britain's future in the EU. The Brexit vote has disrupted the certainty American officials came to expect from 10 Downing Street on issues from fighting the Islamic State group to a successful trade relationship.

American officials have urged calm in the days following Thursday's "leave" vote. "It is absolutely essential that we stay focused on how in this transitional period, nobody loses their head, nobody goes off half-cock, people don't start moving on scatter-brained or revengeful premises," Kerry said Monday.

But finding a common position with the Germans on major issues could be a challenge. Washington and Berlin differ on fiscal policy over Greece’s economic crisis and Germany has not actively engaged in combat against the Islamic State group despite the White House's calls for help. However, Germany's clout in the EU and strong economy make it an attractive partner as Britain's role looks uncertain.

Kerry is also scheduled this week to meet with Cameron in London. Cameron, who campaigned for a “remain” vote, said he would resign after the ruling Conservative Party appoints a new leader. A new leader is expected as soon as September.