To those who study or even practiced espionage, U.S. spying on allies is hardly new.
Friendly nations spy on one another, if only for this reality: a friend today may not be a friend tomorrow, experts say.
"Even among friends, a lot of espionage takes place, and some of that espionage is targeted against threats to national security," said Charles Kupchan, international affairs professor at Georgetown University and a senior fellow at the Council on Foreign Relations.
"Then there is more mundane day-to-day intelligence gathering, which is focusing on intelligence that would be relevant to American statecraft: who is likely to be the next foreign minister, what's Germany's position on negotiations with Iran?" Kupchan said.
The mutual spying is "common knowledge" among practitioners and scholars, sometimes confirmed years later with a disclosure, Kupchan said.
"It's been going on for centuries," he said.
Embassies can be an arm of such espionage. Countries allow the missions to exchange information formally but they're used to gather intelligence covertly, too, said Peter Earnest, who worked for the CIA for 36 years, including about 25 years in the agency's clandestine service.
"I think there's a degree of hypocrisy among the Europeans to say, 'Oh, my gosh, the Americans are spying!' Well, so are they," said Earnest, the founding executive director of the International Spy Museum in Washington.
The former spy was referring to the uproar across Europe in the wake of news accounts that the U.S. National Security Administration has been spying on the continent: on German Chancellor Angela Merkel's official cellphone; on millions of France's phone calls; and on millions of Spain's phone calls and its politicians and officials.
The NSA also has eavesdropped on the Mexican government and hacked the public e-mail account of former President Felipe Calderon and his presidency's e-mail domain that also was used by Cabinet members, according to German news magazine Der Spiegel.
Most of the news accounts are relying on documents provided by former NSA contractor Edward Snowden.
Glaring spotlight on intelligence committee
"It's the leak that keeps on giving on damage," Earnest said. "I'm glad I'm not in the community right now. It must be a nightmare." He retired from the Central Intelligence Agency in 1994.
In describing the espionage among friends, Earnest referred to a famous quote by 19th century British statesman Henry Temple, or Lord Palmerston, who stated in Parliament: "We have no eternal allies and we have no perpetual enemies. Our interests are eternal and perpetual, and those interests it is our duty to follow."
Before Congress on Tuesday, Director of National Intelligence James Clapper acknowledged as a "fundamental given" that the United States gathers intelligence on foreign leaders.
When asked by House Intelligence Committee Chairman Mike Rogers whether U.S. allies spy against U.S. leaders, Clapper replied: "Absolutely."
"We do not spy on anyone except for valid foreign intelligence purposes and we only work within the law. Now, to be sure, on occasion, we've made mistakes -- some quite significant," Clapper told the House committee reviewing the agency's surveillance activities. Clapper has worked in intelligence services for 50 years.
In other testimony to the committee, Army Gen. Keith Alexander, director of the NSA, said media outlets misinterpreted the leaked documents. Some of the metadata on phones calls in ally countries came from those countries, and the remaining metadata was collected legally by the NSA, he said.
Historians readily cite instances of U.S. spying on allies, including bugging of conversations in San Francisco, where the United Nations charter was drawn up in 1945, said David Barrett, a political science professor at Villanova University who has written about U.S. intelligence history.
"I have to say I'm not very surprised," Barrett said of U.S. surveillance on friends.
'Horrified by what they had learned'
During the Cold War in 1960, two NSA employees, William Martin and Bernon Mitchell, defected to the Soviet Union and publicly revealed how the United States was spying on not just rivals, but allies, too.
The NSA employees were like Snowden in that they became dismayed to learn of such spying, Barrett said. Snowden is now residing in Russia under political asylum.
"There was a lot in common between them and Edward Snowden. They were horrified by what they learned. To me, they're arrogant. And they wanted to do something about it, and so they released information," Barrett said.
Former U.S. Vice President Dick Cheney spoke in general terms about American spying prowess, noting how the nation enjoys "a fantastic intelligence capability worldwide against all kinds of potential issues and concerns," especially in the aftermath of the September 11, 2001, attacks.