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Former intelligence worker Edward Snowden revealed himself as the source of documents outlining a massive effort by the NSA to track cell phone calls and monitor the e-mail and Internet traffic of virtually all Americans. He says he just wanted the public to know what the government was doing. "Even if you're not doing anything wrong, you're being watched and recorded," he said. Snowden has been granted temporary asylum in Russia after initially fleeing to Hong Kong. He has been charged with three felony counts, including violations of the U.S. Espionage Act, over the leaks.

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.

Starting in 1932, the U.S. Public Health Service studied untreated syphilis in black men who thought they were getting free health care. The patients weren't told of their affliction or sufficiently treated. Peter Buxtun, who worked for the Public Health Service, relayed information about the Tuskegee syphilis experiment to a reporter in 1972, which halted the 40-year study. His testimony at congressional hearings led to an overhaul of the Health, Education and Welfare rules concerning work with human subjects. A class-action lawsuit was settled out-of-court for $10 million, with the U.S. government promising free medical care to survivors and their families. Here, participants talk with a study coordinator.

In 2005, retired deputy FBI director Mark Felt revealed himself to be the whistle-blower "Deep Throat" in the Watergate scandal. He anonymously assisted Washington Post reporters Carl Bernstein and Bob Woodward with many of their stories about the Nixon administration's cover-up after the June 1972 break-in at the Democratic National Committee headquarters. The stories sparked a congressional investigation that eventually led to President Nixon's resignation in 1974. The Post won a Pulitzer Prize for its coverage. Felt was convicted on unrelated conspiracy charges in 1980 and eventually pardoned by President Ronald Reagan before slipping into obscurity for the next quarter-century. He died in 2008 at age 95.

Mordechai Vanunu, who worked as a technician at Israel's nuclear research facility, leaked information to a British newspaper and led nuclear arms analysts to conclude that Israel possessed a stockpile of nuclear weapons. Israel has neither confirmed nor denied its weapons program. An Israeli court convicted Vanunu in 1986 after Israeli intelligence agents captured him in Italy. He was sentenced to 18 years in prison. Since his release in 2004, he has been arrested on a number of occasions for violating terms of his parole.

President Ronald Reagan addresses the media in 1987, months after the disclosure of the Iran-Contra affair. A secret operation carried out by an American military officer used proceeds from weapons sales to Iran to fund the anti-communist Contras in Nicaragua and attempted to secure the release of U.S. hostages held by Iran-backed Hezbollah in Lebanon. Mehdi Hashemi, an officer of Iran's Islamic Revolutionary Guards Corps, leaked evidence of the deal to a Lebanese newspaper in 1986. Reagan's closest aides maintain he did not fully know, and only reluctantly came to accept, the circumstances of the operation.

Tobacco industry executive Jeffrey Wigand issued a memo to his company in 1992 about his concerns regarding tobacco additives. He was fired in March 1993 and subsequently contacted by "60 Minutes" and persuaded to tell his story on CBS. He claimed that Brown & Williamson knowingly used additives that were carcinogenic and addictive and spent millions covering it up. He also testified in a landmark case in Mississippi that resulted in a $246 billion settlement from the tobacco industry. Wigand has received public recognition for his actions and continues to crusade against Big Tobacco. He was portrayed by Russell Crowe in the 1999 film "The Insider."

For 10 years, Frederic Whitehurst complained mostly in vain about practices at the FBI's world-renowned crime lab, where he worked. His efforts eventually led to a 1997 investigation that found lab agents produced inaccurate and scientifically flawed testimony in major cases, including the Oklahoma City and World Trade Center bombings. The Justice Department recommended major reforms but also criticized Whitehurst for "overstated and incendiary" allegations. He also faced disciplinary action for refusing to cooperate with an investigation into how some of his allegations were leaked to a magazine. After a yearlong paid suspension he left the bureau in 1998 with a settlement worth more than $1.16 million.

FBI whistle-blower Coleen Rowley accused the bureau of hindering efforts to investigate a suspected terrorist that could have disrupted plans for the September 11, 2001, terror attacks. In 2002 she fired off a 13-page letter to FBI Director Robert Mueller and flew to Washington to hand-deliver copies to two members of the Senate Intelligence Committee and meet with committee staffers. The letter accused the bureau of deliberately undermining requests to look into Zacarias Moussaoui, the only person convicted in the United States of playing a role in the attacks. She testified in front of Congress and the 9/11 Commission about the FBI's mishandling of information. Rowley was selected as one of Time magazine's People of the Year in 2002, along with whistle-blowers Sherron Watkins of Enron and Cynthia Cooper of WorldCom.

Sherron Watkins, a former vice president at Enron, sent an anonymous letter to founder Kenneth Lay in 2001 warning him the company had accounting irregularities. The memo eventually reached the public and she later testified before Congress about her concerns and the company's wrongdoings. More than 4,000 Enron employees lost their jobs, and many also lost their life savings, when the energy giant declared bankruptcy in 2001. Investors lost billions of dollars. An investigation in 2002 found that Enron executives reaped millions of dollars from off-the-books partnerships and violated basic rules of accounting and ethics. Many were sentenced to prison for their roles in the Enron scandal.

Cynthia Cooper and her team of auditors uncovered massive fraud at WorldCom in 2002. They found that the long-distance telephone provider had used $3.8 billion in questionable accounting entries to inflate earnings over the past five quarters. By the end of 2003, the total fraud was estimated to be $11 billion. The company filed for bankruptcy protection and five executives ended up in prison. Cooper started her own consulting firm and told her story in the book "Extraordinary Circumstances: The Journey of a Corporate Whistleblower."

In 2003, federal air marshal Robert MacLean anonymously tipped off an MSNBC reporter that because of budget concerns, the TSA was temporarily suspending missions that would require marshals to stay in hotels just days after they were briefed about a new "potential plot" to hijack U.S. airliners. The news caused an immediate uproar on Capitol Hill and the TSA retreated, withdrawing the scheduling cuts before they went into effect. MacLean was later investigated and fired for the unauthorized disclosure of "sensitive security information."

Joe Darby is the whistle-blower behind the Abu Ghraib prison abuse scandal in Iraq. He says he asked Army Reserve Spc. Charles Graner Jr. for photos from their travels so he could share them with family. Instead, he was given photos of prisoner abuse. Darby eventually alerted the U.S. military command, triggering an investigation and global outrage when the scandal came to light in 2004. Graner was sentenced to 10 years in prison for his part in the abuse. He was released in 2011 after serving 6½ years of his sentence. The military and members of Darby's own family ostracized him, calling him a traitor. Eventually he and his wife had to enter protective custody.

The New York Times reported in 2005 that in the months after the September 11, 2001, attacks, President George W. Bush authorized the U.S. National Security Agency to eavesdrop without a court warrant on people in the United States, including American citizens, suspected of communicating with al Qaeda members overseas. The Bush administration staunchly defended the controversial surveillance program. Russ Tice, an NSA insider, came forward as one of the anonymous sources used by the Times. He said he was concerned about alleged abuses and a lack of oversight. Here, President Bush participates in a conversation about the Patriot Act in Buffalo, New York, in April 2004.

Army Pfc. Bradley Manning was convicted July 30 of stealing and disseminating 750,000 pages of classified documents and videos to WikiLeaks, and the counts against him included violations of the Espionage Act. He was found guilty of 20 of the 22 charges but acquitted of the most serious charge -- aiding the enemy. Manning is set to speak in his defense when he takes the stand during the sentencing phase of his court-martial on Wednesday, August 14. He could face up to 90 years in prison if the judge imposes the maximum sentence.

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Editor's note: Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" and "Governing America."

(CNN) -- As the story about the National Security Agency surveillance continued to unfold last week, some of President Obama's supporters, as well as some of his Republican critics, were quick to jump to his defense.

Chairman of the House of Representatives Intelligence Committee Mike Rogers warned that the allegations about the NSA were wrong. "They are seeing three or four pieces of a thousand-piece puzzle and trying to come to a conclusion."

Speaking before a congressional committee, Director of National Intelligence James Clapper said the monitoring of calls by 35 world leaders was just about spying, something that every country did and so there was nothing to be worked up about. "Some of this reminds me of the classic movie 'Casablanca': 'My God, there's gambling going on here,'" Clapper said.

In a period of crippling partisan warfare that continually brings Washington to a standstill, the leadership of both parties seem to have easily reached bipartisan agreement that the existing national security programs should be left alone.

But these arguments miss the importance of accountability in our national security operations. The notion that citizens should just trust the government to do the right thing on national security poses too many dangers.

The United States has a long history of national security agencies, sometimes with presidential concurrence, misusing their authority and power to harass American citizens. This was the case in the 1960s, when Democrats and Republicans used government institutions to intimidate and harass social activists who were fighting for causes such as civil rights and to protest the war in Vietnam.

With the approval of Attorney General Robert Kennedy, the FBI obsessively wiretapped the Rev. Martin Luther King Jr., with the goal of finding evidence of the role of communism in the civil rights movement. What it found instead was information about his personal life as well as the background of top advisers that could be used against the movement if its demands caused too many problems. FBI Director J. Edgar Hoover leaked information to the press and to King's opponents.

A congressional report later found that the program sought to "discredit Dr. King and to 'neutralize' him as the leader of the civil rights movement." Presidents Lyndon Johnson and Richard Nixon agreed to have authorities bug and infiltrate anti-war activists. The administration spread damaging allegations and information to the media and to supporters of the war with the goal of rendering the activists illegitimate, and nurtured distrust and animosity within the movement so members would turn against each other.

In 1975 and 1976, Idaho Sen. Frank Church conducted shocking hearings into the operations of the CIA and published a detailed report that revealed the agency had been secretly engaged in activities such as the attempted assassination of foreign leaders and illegal intelligence gathering of American citizens. Congress imposed new regulations and created a court to monitor their activities. In the end the regulations proved to be weak and since 9/11 they have essentially been rendered useless.

After the reforms undertaken in light of the abuses of the 1960s and 1970s, intelligence agencies continued their expansion, part of the permanent national security apparatus the United States had established. "The vast secrecy system shows no signs of receding," Sen. Patrick Moynihan wrote in the late 1990s. Then, after 9/11, the power of the government to conduct surveillance greatly expanded beyond anything the senator from New York could have anticipated.

The United States is not alone in facing these risks. One of the reasons Germans have been so sensitive to the recent revelations is their own history of how surveillance has been used aggressively, and violently, to target their own citizens.

Even if the NSA officials play by the rules and regulate themselves, their ability to contain information that could be enormously damaging to the United States and to individual citizens is greatly diminishing in the current era. They no longer are in full control, whatever their intentions might be.

The stories about WikiLeaks as well as the impact of Edward Snowden have shown that the control of classified information is becoming extraordinarily difficult. It is foolhardy to assume that government officials will be able to contain all the data they gather and we must expect that more leaks will occur. When those leaks occur it is also remarkably easy for the information to quickly reach the public via the Internet.

Given that the risks of leaks will continue to remain great, the most effective measures that Congress can take will be to work on regulatory checks at the point of gathering to make sure that national intelligence agents don't sweep up innocent civilians and important allied leaders in their sweeps.

The worst outcome of the NSA revelations would be for Congress and the media to descend into a media-frenzy focusing primarily on the President's statements about what he knew. What would be much more useful to the nation would be for the executive and legislative branches to conduct a thorough review of how the NSA conducts its business and to put into place measures that will prevent unneeded activities that cause more harm than good, and which unnecessarily infringe the civil liberties of innocent people.

In June, when the revelations first broke, Senate Majority Leader Harry Reid said, "Everyone should just calm down and understand this isn't anything that is brand new." Whether or not this is brand new, the time has come for Congress to take a serious look at what the NSA is doing and how to impose needed regulations on what the agency can do. Representative James Sensenbrenner (R-Wisconsin) and Senator Patrick Leahy (D-VT) have started the debate with a bill with sweeping reforms, including a special court to monitor intelligence gathering. Reid and Speaker Boehner have offered cautious support for a review of the program.

In the long run, such steps would have the potential to strengthen trust and confidence in the NSA, and allow it to do the work that is needed to secure the nation and our allies against the threats the United States continually faces.

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The opinions expressed in this commentary are solely those of Julian Zelizer.