Some did it for the money, some did it for idealism, others didn't do it at all. The U.S. has seen a number of high profile leak scandals including the Pentagon Papers during the administration of President Richard Nixon. Click through to see more high-profile intelligence leaking cases. 
Military analyst Daniel Ellsberg leaked the 7,000-page Pentagon Papers in 1971. The top-secret documents revealed that senior American leaders, including three presidents, knew the Vietnam War was an unwinnable, tragic quagmire. Further, they showed that the government had lied to Congress and the public about the progress of the war. Ellsberg surrendered to authorities and was charged as a spy. During his trial, the court learned that President Richard Nixon's administration had embarked on a campaign to discredit Ellsberg, illegally wiretapping him and breaking into his psychiatrist's office. All charges against him were dropped. Since then he has lived a relatively quiet life as a respected author and lecturer. 
Jonathan Pollard is a divisive figure in U.S.-Israel relations. The former U.S. Navy intelligence analyst was caught spying for Israel in 1985 and was sentenced in 1987 to life imprisonment. Israeli Prime Minister Benjamin Netanyahu has publicly called for President Barack Obama to release Pollard after Pollard's wife appealed to Netanyahu. 
Wen Ho Lee was a scientist at the Los Alamos Laboratory in New Mexico who was charged with 59 counts of downloading classified information onto computer tapes and passing it to China. Lee eventually agreed to plead guilty to a since count of mishandling classified information after prosecutors deemed their case to be too weak. He was released after nine months in solitary confinement. Lee later received a $1.6 million in separate settlements with the government and five news agencies after he sued them, accusing the government of leaking damaging information about him to the media. 
Members of the Bush administration were accused retaliating against Valerie Plame, pictured, by blowing her cover in 2003 as a U.S. intelligence operative, after her husband, former Ambassador Joe Wilson, wrote a series of New York Times op-eds questioning the basis of certain facts the administration used to make the argument to go to war in Iraq. 
In 2007, Lewis "Scooter" Libby, Vice President Dick Cheney's former chief of staff, was convicted on charges related to the leak of the identity of CIA operative Valerie Plame. Libby was convicted of obstruction of justice and perjury in connection with the case. His 30-month sentence was commuted by President George W. Bush. Cheney told a special prosecutor in 2004 that he had no idea who leaked the information. 
Aldrich Ames, a 31-year CIA employee, pleaded guilty to espionage charges in 1994 and was sentenced to life in prison. Ames was a CIA case worker who specialized in Soviet intelligence services and had been passing classified information to the KGB since 1985. U.S. intelligence officials believe that information passed along by Ames led to the arrest and execution of Russian officials they had recruited to spy for them. 
Robert Hanssen pleaded guilty to espionage charges in 2001 in return for the government not seeking the death penalty. Hanssen began spying for the Soviet Union in 1979, three years after going to work for the FBI and prosecutors said he collected $1.4 million for the information he turned over to the Cold War enemy. In 1981, Hanssen's wife caught him with classified documents and convinced him to stop spying, but he started passing secrets to the Soviets again four years later. In 1991, he broke off relations with the KGB, but resumed his espionage career in 1999, this time with the Russian Intelligence Service. He was arrested after making a drop in a Virginia park in 2001. 
John Walker ran a father and son spy ring, passing classified material to the Soviet Union from 1967 to 1985. Walker was a Navy communication specialist with financial difficulties when he walked into the Soviet Embassy and sold a piece of cyphering equipment. Navy and Defense officials said that Walker enabled the Soviet Union to unscramble military communications and pinpoint the location of U.S. submarines at all times. As part of his plea deal, prosecutors promised leniency for Walker's son Michael Walker, a former Navy seaman. 
Pfc. Bradley Manning is an Army intelligence specialist who is charged with passing along classified material to WikiLeaks, a group that facilitates anonymous leaking of secret information. Manning pleaded guilty to 10 of the 22 charges against him and could face 20 years in prison. But he pleaded not guilty to the most serious charge, aiding U.S. enemies, which carries a potential life sentence. 
Former intelligence contractor Edward Snowden revealed himself as the leaker of details of U.S. government surveillance programs run by the U.S. National Security Agency to track cell phone calls and monitor the e-mail and Internet traffic of virtually all Americans. Snowden fled to Hong Kong after copying one last set of documents and telling his boss he needed to go away for medical treatment. - Michael Hayden: Snowden will likely be most damaging leaker in American history
- He says the large trove of data reveals how America collects much of its intelligence
- Hayden says U.S. economic rivals will use it to disadvantage American companies
- He says other nations will doubt whether the U.S. can do anything in secret
Editor's note: Gen. Michael V. Hayden, a former NSA director who was appointed by President George W. Bush as CIA director in 2006 and served until February 2009, is a principal with the Chertoff Group, a security consulting firm. He serves on the boards of several defense firms and is a distinguished visiting professor at George Mason University.
(CNN) -- Edward Snowden will likely prove to be the most costly leaker of American secrets in the history of the Republic.
I know that we have had our share of spies.
Benedict Arnold was bent on betraying the garrison at West Point to the British during the Revolution. Klaus Fuchs and Julius and Ethel Rosenberg ferreted out nuclear secrets for the Russians. Aldrich Ames and Robert Hanssen identified American penetrations for ultimate execution by the Soviets.
We have also had our share of leakers.
Daniel Ellsberg copied thousands of pages of documents related to the Vietnam War. Bradley Manning is accused of indiscriminately scoured the Defense Department's SIPRNET (Secret Internet Protocol Router Network) for all manner of military reports and diplomatic cables.
But Snowden is in a class by himself.
The secrets that Arnold wanted to betray fit into the heel of the boot of his British case officer. The "atom bomb" spies reported out using secret ink. Ellsberg was limited to the number of documents he could physically Xerox. Manning, although fully empowered by digital media, had access only to a secret level network housing largely tactical information.
Snowden fled to China with several computers' worth of data from NSANET, one of the most highly classified and sensitive networks in American intelligence. The damage is potentially so great that NSA has taken one of its most respected senior operations officers off mission tasks to lead the damage assessment effort.
In general terms, it's already clear Snowden's betrayal hurts in at least three important ways.
First, there is the undeniable operational effect of informing adversaries of American intelligence's tactics, techniques and procedures. Snowden's disclosures go beyond the "what" of a particular secret or source. He is busily revealing the "how" of American collection.
The Guardian newspaper's Glenn Greenwald, far more deserving of the Justice Department's characterization of a co-conspirator than Fox's James Rosen ever was, claims that Snowden has documents that comprise "basically the instruction manual for how the NSA is built. ... [To prove] what he was saying was true, he had to take ... very sensitive, detailed blueprints of how the NSA does what they do."
Greenwald has disputed the notion that he aided Snowden, telling David Gregory on NBC's "Meet the Press": "The assumption in your question, David, is completely without evidence, the idea I've aided and abetted him in any way."
And Michael Clemente, Fox News' executive vice president of news, has said, "we are outraged to learn ... that James Rosen was named a criminal co-conspirator for simply doing his job as a reporter."
Greenwald: Snowden is 'the classically responsible whistle-blower'
Reporter: Snowden a responsible whistleblower 
Will Snowden release more intelligence? Absent "rogue" U.S. action to silence him, Snowden has promised not to reveal this data, but there are already reports of counterterrorism targets changing their communications patterns. And I would lose all respect for China's Ministry of State Security and Russia's FSB if they have not already fully harvested Snowden's digital data trove.
As former director of CIA, I would claim that the top 20% of American intelligence -- that exquisite insight into an enemy's intentions -- is generally provided by human sources. But as a former director of NSA, I would also suggest that the base 50% to 60% of American intelligence day in and day out is provided by signals intelligence, the kinds of intercepted communications that Snowden has so blithely put at risk.
But there is other damage, such as the undeniable economic punishment that will be inflicted on American businesses for simply complying with American law.
Others, most notably in Europe, will rend their garments in faux shock and outrage that these firms have done this, all the while ignoring that these very same companies, along with their European counterparts, behave the same way when confronted with the lawful demands of European states.
The real purpose of those complaints is competitive economic advantage, putting added burdens on or even disqualifying American firms competing in Europe for the big data and cloud services that are at the cutting edge of the global IT industry. Or, in the case of France, to slow negotiations on a trans- Atlantic trade agreement that threatens the privileged position of French agriculture, outrage more based on protecting the production of cheese than preventing any alleged violation of privacy.
The third great harm of Snowden's efforts to date is the erosion of confidence in the ability of the United States to do anything discreetly or keep anything secret.
Manning's torrent of disclosures certainly caused great harm, but there was at least the plausible defense that this was a one-off phenomenon, a regrettable error were aggressively correcting.
Snowden shows that we have fallen short and that the issue may be more systemic rather than isolated. At least that's what I would fear if I were a foreign intelligence chief approached by the Americans to do anything of import.
Snowden seems undeterred by any of these consequences. After all, he believes he is acting for a higher good -- an almost romantic attachment to the merits of absolute transparency -- and he seems indifferent to the legitimacy of any claims of national security.
The appropriate balance between liberty and security has bedeviled free peoples, including Americans, for centuries. But it takes a special kind of arrogance for this young man to believe that his moral judgment on the dilemma suddenly trumps that of two (incredibly different) presidents, both houses of the U.S. Congress, both political parties, the U.S. court system and more than 30,000 of his co-workers.
Arrogant or not, Snowden has thrust into public view sensitive and controversial espionage activities. So what of his facts, fictions and fears and of the national debate that he claims he intended to stimulate?
More on this in following columns.
Another view: Edward Snowden is a hero
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The opinions expressed in this commentary are solely those of Michael Hayden.
