As the scandal over the United States spying on friends and allies expands beyond German Chancellor Angela Merkel’s cell phone to perhaps dozens of other countries, one question lingering in the background is: Why spy on friends anyway?

The basic answer, some national intelligence and security experts say, is that relations among countries are essentially based on interests, and no matter how friendly countries may be, their interests are rarely exactly the same.

“We and Germany don’t always see eye-to-eye on some important issues,” says James Lewis, director of the Technology and Public Policy Program at the Center for Strategic and International Studies in Washington. “One way to reassure yourself about the direction an ally like Germany is heading on one of those issues is to know what Germany is saying.”

In recent years the US and Germany have had their differences on issues ranging from technology trade with Iran to how to counter Chinese cyberespionage, Mr. Lewis notes. And beyond such areas of disagreement, the US would also have an interest in knowing Germany’s thinking on issues with global implications where it is playing a central role, he adds.

“Germany pretty much kept Europe afloat in the [2008] financial crisis,” says Lewis, citing an example of a fast-moving global challenge where the US would have wanted as much information as possible.

“If Germany had decided to stop supporting Greece, for example, it would have had major global repercussions,” he adds. “It would have been important for us to know that.”

It was French World War II statesman Charles de Gaulle who famously quipped that “No nation has friends, only interests,” but in the current uproar over National Security Agency spying on American allies, France is taking a different tack.

French President Francois Hollande, saying spying among friends is “unacceptable,” on Friday joined Chancellor Merkel in demanding talks with the US aimed at setting new rules for the intelligence-gathering road.

At a meeting of European Union leaders in Brussels, Merkel said relations with the US had been “severely shaken” by the spying allegations, and said damaged trust would have to be rebuilt through the imposition of new rules governing surveillance activities.

“Obviously words will not be sufficient,” she said.

That the NSA could even consider tapping into – “tasking,” in the community’s jargon – Merkel’s cell phone reflects the reality both of how relatively easy it has become to do that, and of how information-gathering and information-storing capabilities have exploded over the past decade.

With intelligence capabilities expanding in ways unimagined just a matter of years ago, “It becomes an accretive growth process,” says Allan Friedman, research director of the Center for Technology Innovation at Washington’s Brookings Institution.

Asked how this process could end up with Merkel’s cell phone being “tasked,” Mr. Friedman says, “The prevailing thinking is, ‘We got that much [information] this year, what can we do to get more next year?’ ”

That “thinking” stood out in a classified NSA memo provided to The Guardian newspaper by NSA leaker Edward Snowden, in which the agency asked officials in its “customer” departments – the White House, State Department, Pentagon, and others – to make their “Rolodexes” available to the intelligence agency.

“Such ‘Rolodexes’ may contain contact information for foreign political or military leaders, to include direct line, fax, residence and cellular numbers,” the memo said. To convince officials of the usefulness of such an exercise, the memo cited the example of one official who turned over a trove of previously uncollected phone numbers.

“In one recent case, a US official provided NSA with 200 phone numbers to 35 world leaders,” the 2006 memo notes. “Despite the fact that the majority is probably available via open sources, [NSA intelligence production centers] have noted 43 previously unknown phone numbers.” Those numbers and others culled from officials “have subsequently been tasked,” the memo says.

The spying scandal and presumably the leaked memo in particular prompted President Obama’s chief assistant for homeland security and counterterrorism, Lisa Monaco, to pen an opinion piece in Friday’s USA Today in which she seeks to reassure American allies that US intelligence gathering has not run amok.

Noting that “Today’s world is highly interconnected, and the flow of large amounts of data is unprecedented,” she says Mr. Obama has ordered a review of surveillance capabilities – including “with respect to our foreign partners” – because “We want to ensure we are collecting information because we need it and not just because we can.

That point stands out to some national security experts as a clue to deciphering how the US came to intercept the personal cell phones and e-mail accounts of friendly world leaders.

“This is an important piece of evidence that our technological capabilities have far outstripped our thinking about how we should use those capabilities to best advance US national security,” says Michael Desch, a US foreign policy and international security expert at the University of Notre Dame in Indiana.

Neither Merkel nor former Mexican President Felipe Calderon, whose e-mail account was intercepted, according to leaked documents, could really have been “shocked’ that their communications were the target of US surveillance, Dr. Desch says. But he adds that the “diplomatic fallout” from the surveillance controversy should lead the US to think about whether “even if we have the technical capability to target these leaders, … it is in our interest to do so.”

But others insist that friends have been spying on friends for a long time – former Secretary of State Madeleine Albright asserts that the French spied on her when she was the top US diplomat – and that the practice is unlikely to suddenly stop.

“There is this notion that it’s OK to eavesdrop on enemies but not on your friends,” says Brookings’s Friedman. “But it’s really a polite fiction.” And it’s not just the US doing it, he adds.

“No one with clearance would think that members of the EU [European Union] don’t try to find out about each other by using their intelligence services,” he says.

Another issue in an increasingly globalized world is spying on friends or trading partners to gain economic advantage – which China is often accused of – but such aims are not a big part of US information gathering, some experts say.

“The US really doesn’t do that, for the most part we’re looking for the big things,” says CSIS’s Lewis, “things like: is Germany going to continue supporting Greece, evidence of major corruption involving corporations and officials, or cases related to [WMD] proliferation,” he says.

In Lewis’s estimation, Europeans aren’t overly concerned about US economic espionage, in large part because they see the “hubris” of Americans who believe they are the best in many domains and don’t need to spy on others to advance. “It’s easy for the Europeans to believe in American arrogance,” he says.

What may not be so easy for Europeans and others will be to believe that US spying on friends and allies has stopped. Despite what Ms. Monaco of the White House says, Lewis notes that “it’s become so easy to do all this” information gathering and communications surveillance that “it’s hard for people to resist temptation.”

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That said, he adds, the means are there to repair relations.

“You say, ‘OK, we’ll back off,’ and you mean it,” Lewis says. “Once you make a commitment, that’s significant, and you do what you have to to honor it.”