WASHINGTON -- WASHINGTON (AP) — When President Barack Obama visited Berlin in June, German Chancellor Angela Merkel made a point of showing him a balcony in her office overlooking train tracks that crossed the border of her once-divided country — a symbol of her upbringing on the east side of the divide, where eavesdropping by secret police was rampant during the Cold War.

The private moment between the two leaders underscores the degree to which Merkel's personal history has influenced her outrage over revelations that the National Security Agency was monitoring her communications. The secret spying threatens to damage the close relationship between Obama and Merkel, which, until now, has been defined by candor and trust.

"We are very sensitive to the fact that she comes from the East, and that brings with it a historical perspective on surveillance that is quite powerful," said Ben Rhodes, Obama's deputy national security adviser. He said while the White House hopes the strength of Obama's relationship with Merkel will allow them to weather the current controversy, "it also clearly makes it more difficult when she is surprised by these types of revelations."

Reports based on leaks from former NSA systems analyst Edward Snowden suggest the U.S. has monitored the telephone communications of 35 foreign leaders. The fact that Merkel was among them has been particularly troubling to many in Europe and on Capitol Hill, given her status as a senior stateswoman, the leader of Europe's strongest economy, and a key American ally on global economics, Iranian nuclear negotiations and the Afghanistan war.

Obama, in a phone call to Merkel last week, said the U.S. was not currently monitoring her communications and had no plans to do so in the future. But those assurances appeared to do little to placate the German leader, who said trust with the U.S. "has to be built anew" and there must be no "spying among friends."

A U.S. official said Obama was only made aware that the NSA was monitoring Merkel in recent weeks, after the White House launched a broader review of surveillance programs following Snowden's revelations. The official was not authorized to discuss the matter by name and insisted on anonymity.

In Merkel, Obama has found a leader who shares his businesslike, cut-to-the-chase approach to leader-level diplomacy. Both eschew the staged formality of international summits and sometimes can be seen as cold and brusque by some of their counterparts.

"She's got a style and mannerism that feels familiar to him," said Tommy Vietor, who until March was Obama's National Security Council spokesman. "She's no-nonsense, but she's also really effective and gets things done."

First elected chancellor in 2005, Merkel was already a formidable player on the world stage when much of Europe became enthralled by an upstart American politician named Barack Obama. He was particularly popular in Germany and scheduled a stop there during an overseas trip ahead of the 2008 election. He hoped to speak at Berlin's Brandenburg Gate, the site of famous speeches by past American presidents.

But Merkel made it clear that she disapproved of plans for a politician to hold a campaign rally at the site, a landmark symbol of the Cold War. For Obama, it was an early lesson in Merkel's firmness. His campaign reversed course and another site was selected for the speech.

Obama finally got his chance to speak at the Brandenburg Gate this summer, and Merkel was there to introduce him. She ended her remarks by referring to Obama with a German word for "you" that is typically reserved for friendly visitors, sparking surprised laughter from the crowd.

In the years between Obama's trips to Berlin, he and Merkel have attempted to inject some warmth into their business relationship. During Merkel's 2011 visit to Washington, the two leaders hit the town for dinner at a high-end restaurant, an unusual overture by Obama. A few days later, he hosted Merkel at the White House for a formal state dinner, where he awarded the chancellor the Presidential Medal of Freedom, the highest U.S. honor bestowed upon civilians.

Despite their strong working relationship, Obama and Merkel have had deep policy differences, particularly over the response to the European debt crisis. During the depths of the crisis in 2011 and 2012, Merkel pushed for fiscal austerity on the continent, while Obama and many European leaders backed American-style stimulus.

Both leaders were driven by domestic concerns: for Obama, his looming re-election campaign, and for Merkel, the German public's unease with bailing out its fiscally irresponsible neighbors.

"During the debt crisis, Merkel just felt like Barack Obama never fully understood what was at stake — which was Europe," said Heather Conley, director of the Europe program at the Center for Strategic and International Studies. "And I think the president felt like Angela Merkel didn't quite understand the impact the European economy could have on his re-election efforts."

The NSA surveillance disclosures already had revealed another policy split between Obama and Merkel even before the revelations that the U.S. was monitoring the German leader's phone.

When Obama visited Germany earlier this year, Merkel was under election-year pressure from her privacy-protective constituents to condemn NSA programs that swept up phone and email records, including those of Europeans. She raised the issue with Obama both publicly and privately, casting a shadow over what had been expected to be a tension-free visit.

It's unclear whether the NSA spying disclosures will still cast a pall over Obama and Merkel's relationship when they next meet, likely next year during the international summits both regularly attend. But what is known is that the two leaders are stuck with each other for a while. Last month, Merkel convincingly won a third term as chancellor, meaning she'll still be office after Obama leaves the White House in 2016.

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Associated Press writers Robert Reid and Geir Moulson in Berlin contributed to this report.

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Follow Julie Pace on Twitter at http://twitter.com/jpaceDC

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