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The publication of a slew of documents relating to the war in Afghanistan represents one of the largest ever leaks, but what role has illicit information played in US politics?

Perhaps the most famous political scandal in US history also represents the most notorious leak.

The supply of information that "Deep Throat" - later revealed to be FBI deputy director Mark Felt - provided led reporters Bob Woodward and Carl Bernstein to uncover the Watergate scandal.

Without the illicit information it is hard to imagine the full extent of the cover-up over the burglary of the Democratic National Committee offices would have come to light, or that President Richard Nixon would have been forced to resign.

The Pentagon Papers was another leak that had a major impact.

A study of the US military's role in Vietnam (formerly Indochina) from 1945 to 1967, they were supplied to a New York Times journalist in 1971.

The publication, which the Nixon administration tried to stop, revealed details about the way the war had been escalated which seemed to contradict what the public had been told by the previous administration of President Lyndon Johnson.

Indeed, many leaks originate in the world of the military or intelligence, where whistleblowers justify the release of sensitive documents by arguing that their classified status would mean wrongdoing might not be revealed.

The allegations of abuse of detainees by US personnel at Iraq's Abu Ghraib prison had been reported in the media at the beginning of 2004.

But it was only when the photos of the ill-treatment of prisoners were supplied to the 60 Minutes current affairs programme and to the investigative journalist Seymour Hersh that it turned into a full-blown military scandal.

The pictures showed naked prisoners being sexually humiliated and physically abused and were front-page news around the world. Eleven US soldiers were convicted of abusing prisoners.

The scandal also highlighted the death of detainee Manadel al-Jamadi while being interrogated in US custody.

In 2003, journalist Robert Novak reported that Valerie Plame was a CIA agent, ending her career as a covert operative.

The revelation sparked an investigation into how the information had made its way to Novak.

It was alleged by critics that senior White House aides had revealed the information as revenge against Ms Plame's husband, former Ambassador Joseph Wilson, for an article in which he debunked some of the justification for invading Iraq.

White House aide Lewis "Scooter" Libby was subsequently found guilty of lying to the FBI and a grand jury and sentenced to 30 months in prison. He avoided actually going to jail as his sentence was commuted by President George W Bush.

In 2005, leaks to the New York Times helped to reveal that US intelligence was conducting wiretapping without warrants.

Under normal circumstances the National Security Agency was required to obtain a warrant from the special surveillance court before eavesdropping on someone in the US.

But President Bush authorised a special scheme after the 9/11 terror attacks that allowed international phone calls and e-mails to or from the US to be tapped without a warrant, as long as the person or people involved were suspected of having terrorist links.

Critics said that the scheme violated the US constitution and legal safeguards. While maintaining it was legal, the Bush administration dropped the scheme in 2007.