In World War II, the government made sure soldiers had no questions about where they stood when it came to the often fuzzy line between national security and the public's right to know. Each was given a brochure detailing what not to talk about with whom, titled "Loose Lips Sink Ships."
"There are good leaks and bad leaks because there is good secrecy and there is bad secrecy," said Steven Aftergood, director of the Project on Government Secrecy at the Federation of American Scientists.
"Secrecy is good when it conceals information that could be damaging to the country, like design details of advanced military technology, identities of intelligence sources or details of diplomatic negotiations.
"The problem is there is also bad secrecy - information withheld from the public to avoid controversy, evade oversight or other illegitimate reasons. When that kind of information leaks, that's a good leak."
Benjamin Franklin didn't invent the leak, but his participation in one - besides subjecting the founding father to a severe verbal thrashing - further incited the tensions that led to the American Revolution.
Ironically, he was trying to have the opposite effect.
"There has lately fallen into my hands part of a correspondence that I have reason to believe laid the foundation of most if not all our present grievances," Franklin wrote in 1772 to the Massachusetts speaker of the house.
His note was accompanied by a batch of letters written to officials in London, including six by Thomas Hutchinson, the royal governor of Massachusetts. The letters attacked colonial leaders and suggested curtailing their liberties.
The letters had been passed to Franklin, living in England at the time, by an unnamed member of Parliament who was an opponent of Hutchinson's.
Franklin sent them to radical leaders in the colony - asking they not be made public - in hopes of easing growing tensions by showing the colonists that Hutchinson, not England, was the cause of their problems.
When the letters were published, there was an uproar. In Boston, angry colonists began pushing for a recall of Hutchinson. In England, speculation was rampant over who leaked the letters, and accusations were flying. Two men - accusing each other of having leaked the letters - went so far as to engage in a duel. When neither was killed, they agreed to a rematch, at which point Franklin stepped in, writing a letter to The London Chronicle:
"I think it incumbent on me to declare (for the prevention of farther mischief ... ) that I alone am the person who obtained and transmitted to Boston the letters in question ... They were not of the nature of private letters between friends. They were written by public officers to persons in public station ... "
After his admission, Franklin was summoned before the Privy Council for what he thought was a discussion on removing Hutchinson from office.
Instead, in a room known as the "cockpit" - because cockfights had been held there during the reign of Henry VIII - Franklin received what one observer called "a torrent of virulent abuse" from a table-pounding solicitor general named Alexander Wedderburn.
The council rejected the petition to remove Hutchinson from office, but Franklin was removed from his job as American postmaster. Two years later, in 1775, Franklin returned to America to take part in the founding of a new nation.
It is considered the granddaddy of all leaks.
Starting in 1969, Daniel Ellsberg began photocopying a 7,000-page top-secret Pentagon report that documented U.S. policy in Vietnam, and how the American public had been lied to and misled about the war.
In 1971, hoping to turn public sentiment against what he saw as an unjust war, Ellsberg began delivering the documents - first to The New York Times, then The Washington Post and, later, 17 other newspapers as he traveled across the country trying to avoid authorities.
The government tried to stop publication through an injunction - the first time in history that had been done - but it was unsuccessful.
After two weeks on the run, Ellsberg turned himself in. Charged with 12 felonies, he faced a sentence of up to 115 years.
Because no law specifically prohibits supplying classified information to the media, Ellsberg and Anthony Russo, a colleague at the Rand Corp. who had helped him copy the papers, were prosecuted under the Espionage Act of 1917.
First proposed amid the hysteria of World War I, the law made it a crime to disclose national defense secrets to the enemy. Originally, it would have made leaking illegal, but, amid concerns that such a provision would abridge free speech, it was dropped.
Among those spending anxious moments awaiting Ellsberg's trial was President Nixon - even though the Pentagon Papers didn't deal with his term as president.
Still, Nixon wanted to send a message to Ellsberg and others who would leak government secrets - and he wasn't above using leaks to fight the leakers, according to White House tapes: