Revelations that the U.S. has been monitoring the cellphone calls of up to 35 world leaders, including German Chancellor Angela Merkel, have brought into high relief the open-yet-often-unspoken secret that even close allies keep things from one another - and work every angle to find out what's being held back.
So it is that the Israelis recruited American naval analyst Jonathan Pollard to pass along U.S. secrets including satellite photos and data on Soviet weaponry in the 1980s. And the British were accused of spying on U.N. Secretary-General Kofi Annan in the lead-up to the Iraq War. And the French, Germans, Japanese, Israelis and South Koreans have been accused of engaging in economic espionage against the United States.
But now the technology revealed by former National Security Agency analyst Edward Snowden has underscored the incredible new-millennium reach of the U.S. spy agency. And it is raising the question for some allies: Is this still OK?
National Intelligence Director James Clapper, for his part, testified this week that it is a "basic tenet" of the intelligence business to find out whether the public statements of world leaders jibe with what's being said behind closed doors.
What might the Americans have wanted to know from Merkel's private conversations, for example? Ripe topics could well include her thinking on European economic strategy and Germany's plans for talks with world powers about Iran's nuclear program.
There is both motive and opportunity driving the trust-but-verify dynamic in friend-on-friend espionage: Allies often have diverging interests, and the explosion of digital and wireless communication keeps creating new avenues for spying on one another. Further, shifting alliances mean that today's good friends may be on the outs sometime soon.
"It was not all that many years ago when we were bombing German citizens and dropping the atomic bomb on the Japanese," says Peter Earnest, a 35-year veteran of the CIA and now executive director of the International Spy Museum in Washington.
News that the U.S. has tapped foreign leaders' phones was an eye-opener to many - the White House claims that even President Barack Obama wasn't aware of the extent of the surveillance - and has prompted loud complaints from German, French and Spanish officials, among others.
It's all possible because "an explosion in different kinds of digital information tools makes it possible for intelligence agencies to vacuum up a vast quantity of data," says Charles Kupchan, a former Clinton administration official and now a senior fellow of the Council on Foreign Relations. "When you add together the Internet, wireless communications, cellphones, satellites, drones and human intelligence, you have many, many sources of acquiring intelligence."
"The magnitude of the eavesdropping is what shocked us," former French Foreign Minister Bernard Kouchner said in a radio interview. "Let's be honest, we eavesdrop, too. Everyone is listening to everyone else. But we don't have the same means as the United States, which makes us jealous."
Protests aside, diplomats the world around know the gist of the game.
"I am persuaded that everyone knew everything or suspected everything," Russian Foreign Minister Sergey Lavrov said of the reports of U.S. monitoring.
And while prime ministers and lawmakers across Europe and Asia say they are outraged, Clapper told Congress that other countries' own spy agencies helped the NSA collect data on millions of phone calls as part of cooperative counterterror agreements.
Robert Eatinger, the CIA's senior deputy general counsel, told an American Bar Association conference on Thursday that European spy services have stayed quiet throughout the recent controversy because they also spy on the U.S.
"The services have an understanding," Eatinger said. "That's why there wasn't the hue and cry from them."
And another intelligence counsel says the White House can reasonably deny it knows everything about the U.S. spying that's going on.
"We don't reveal to the president or the intelligence committees all of the human sources we are recruiting. ... They understand what the programs are, and the president and chairs of the intelligence committees both knew we were seeking information about leadership intentions," said Robert Litt, general counsel for the Office of the Director of National Intelligence. "They both saw reporting indicating what we were getting if not indicating the source."
Still, Claude Moraes, a British Labor Party politician and member of the European Union delegation that traveled to Washington this week for talks about U.S. surveillance, was troubled by the broad net being cast by U.S. intelligence.
"Friend-upon-friend spying is not something that is easily tolerable if it doesn't have a clear purpose," he said. "There needs to be some kind of justification. ... There is also a question of proportionality and scale."
Obama has promised a review of U.S. intelligence efforts in other countries, an idea that has attracted bipartisan support in Congress.
The United States already has a written intelligence-sharing agreement with Canada, Britain, Australia and New Zealand known as "Five Eyes," and France and Germany might be interested in a similar arrangement.
Paul Pillar, a professor at Georgetown University and former CIA official, worries that a backlash "runs the risk of restrictions leaving the United States more blind than it otherwise would have been" to overseas developments.
The effort to strike the right balance between surveillance and privacy is hardly new.
University of Notre Dame political science professor Michael Desch, an expert on international security and American foreign and defense policies, says the ambivalence is epitomized by Secretary of State Henry L. Stimson's famous line, "Gentlemen do not read each other's mail." Stimson, who served under President Herbert Hoover, shut down the State Department's cryptanalytic office in 1929.
"Leaks about NSA surveillance of even friendly countries such as Brazil, Mexico, and now France make clear that we no longer share Stimson's reticence on this score," Desch said. "While such revelations are a public relations embarrassment, they also reflect the reality that in this day in age, gentlemen do read each other's mail all of the time, even when they are allies."
In fact, a database maintained by the Defense Personnel Security Research Center covering Americans who committed espionage against the U.S. includes activity on behalf of a wide swath of neutral or allied countries since the late 1940s. U.S. citizens have been arrested for conducting espionage on behalf of South Korea, Taiwan, the Philippines, Israel, the Netherlands, Greece, Saudi Arabia, Egypt, Iraq, Jordan, Ghana, Liberia, South Africa, El Salvador and Ecuador, according to the database.
Associated Press Writers Deb Riechmann and Kimberly Dozier contributed to this report.