Throughout history, classified documents have unexpectedly surfaced in public, often due to the act of whistleblowers or political rivals. Some leaks changed the course of history, while others placed governments and people in jeopardy. Below is a list of some of history's most consequential intelligence leaks.

In December 1772, Benjamin Franklin, who was then serving as Britain’s Postmaster General of the American colonies, anonymously received a packet of letters written to a British official by Thomas Hutchinson, the governor of Massachusetts. In the letters, Hutchinson urged Britain to send additional troops to deter rebellious colonists in Boston. Franklin circulated the letters privately, but Samuel Adams had them published in the Boston Gazette in 1773, prompting a scandal that forced Hutchinson to flee the country and fueled tensions that would lead to the Revolutionary War. When three innocent men were accused of leaking the letters, Franklin admitted his role in the affair; he was publicly reprimanded by Parliament and dismissed as Postmaster General.

In 1848, the reporter John Nugent published an unsigned copy of the Treaty of Guadalupe Hidalgo, which would conclude the two-year-long Mexican-American War, in the New York Herald. Questioned by a furious Senate, Nugent refused to reveal his source, beyond insisting it was not a member of the Senate. He was kept under virtual house arrest at the Capitol for a month, but didn’t crack. Ten years later, President James Buchanan gave Nugent a valuable commission to investigate possible development in New Caledonia (now British Columbia). Evidence suggests Buchanan, as secretary of state, was the source of the treaty’s leak.

In June 1971, The New York Times published a series of excerpts from a top-secret Department of Defense report about U.S. involvement in Vietnam between 1945 and 1967. Part of a study commissioned by Secretary of Defense Robert McNamara, the so-called “Pentagon Papers” revealed that four successive presidential administrations had deliberately misled Congress and the American public about the scope, objectives and progress of the Vietnam War. Daniel Ellsberg, a military analyst who opposed the war and had surreptitiously photocopied and leaked the documents, was prosecuted under the 1917 Espionage Act, but the judge later dismissed the charges. Exactly 40 years after the Pentagon Papers leaked, they were declassified and for the first time published in their entirety on the National Archives website.

In mid-1972, five men were arrested for breaking into and trying to bug Democratic National Committee headquarters at the Watergate hotel complex in Washington, D.C. Carl Bernstein and Bob Woodward of the Washington Post were subsequently able to connect the break-in directly to Richard Nixon’s administration, leading to a series of Senate hearings and eventually to Nixon’s resignation in 1974. To get their story, Woodward and Bernstein relied heavily on information from an anonymous informant, dubbed “Deep Throat.” The identity of the man responsible for exposing the biggest political scandal in U.S. history remained a secret for 33 years, until in 2005, the former FBI agent Mark Felt revealed himself as Deep Throat.

In July 2003, Joseph Wilson, who had been a CIA envoy to Niger in 2002, published an op-ed in The New York Times saying George W. Bush’s claim that Iraq attempted to buy uranium from Niger (which the president used to build the case for war) was unsubstantiated. Less than two weeks later, right-wing commentator Robert Novak wrote a column in the Washington Post in which he revealed that Wilson’s wife, Valerie Plame, was a CIA operative. With her cover blown, Plame’s work with the agency was compromised, and Wilson accused the White House of leaking her identity to punish him. An investigation led by a special prosecutor interviewed Bush, Vice President Dick Cheney and other officials, as well as journalists, and in 2007 Lewis “Scooter” Libby, Cheney’s chief of staff, was found guilty on counts of perjury, obstruction of justice and making false statements during the investigation. (Bush later commuted his 30-month sentence.) Libby wasn’t the leak’s source, however: Richard L. Armitage, a former deputy secretary of state, later acknowledged his conversation with Novak likely led to the article outing Plame.

In May 2005, the Sunday Times of London obtained and published a transcript of notes taken in a meeting of Prime Minister Tony Blair’s national security team on July 23, 2002. During the meeting, held nine months before the war in Iraq began, the head of British Secret Intelligence Services (MI6) said his impression from meetings in the United States was that military action was now “inevitable.” According to him, the Bush administration knew that Saddam Hussein didn’t have weapons of mass destruction but had decided to overthrow him by force anyway, and “the intelligence and the facts were being fixed around the policy” in order to publicly justify the invasion. Critics of the war called the Downing Street Memo a “smoking gun” that proved Bush and Blair, his closest ally, made a secret decision to invade Iraq and manipulated the intelligence to support it.

In October 2010, WikiLeaks posted nearly 400,000 classified military documents concerning the Iraq War, a massive info dump that dwarfed its release of some 77,000 documents on the war in Afghanistan several months earlier. WikiLeaks’ founder, the Australian journalist Julian Assange, shared the documents with the press, including the New York Times, Der Spiegel and the Guardian, beforehand. Among the revelations in the so-called Iraq War Logs was evidence that the U.S. military deliberately ignored abuse of detainees by its Iraqi allies, and that there were actually 15,000 more civilian casualties than previously acknowledged. Chelsea Manning, who as Pfc. Bradley Manning had served as a U.S. Army intelligence analyst in Iraq, was later convicted under the Espionage Act for leaking the information. Sentenced to 35 years’ imprisonment, she was pardoned by President Barack Obama in January 2017.  WikiLeaks also released thousands of emails stolen from the computer systems of the Democratic National Committee in 2016. The emails sparked criticism of the committee and Hillary Clinton's campaign. On April 11, 2019, Assange was arrested by British authorities inside the embassy of Ecuador in London. He remains in British prison as he appeals extradition to the United States.

In 2013, Edward J. Snowden, a technical contractor and former CIA employee, leaked classified details of a top-secret National Security Administration (NSA) electronic surveillance program, codenamed PRISM, to the Washington Post and the Guardian. The information, which Snowden obtained while working as a subcontractor for the NSA in Hawaii, revealed that the NSA and FBI were collecting data, including email, chats, videos, photos and social networking information, from ordinary internet users in the U.S. and abroad. Under fire for breach of privacy, President Obama’s administration defended the surveillance program, claiming it helped prevent terrorist attacks. Though some denounced Snowden as a traitor, many others supported his actions, calling him a whistleblower. After federal prosecutors charged Snowden under the Espionage Act, Russia gave him asylum, and he remains there after attempts to gain a presidential pardon proved unsuccessful.

In April 2016, a leak of some 11.5 million files from the database of the Panama-based Mossack Fonseca, the world’s fourth largest offshore law firm, revealed personal financial information about thousands of wealthy individuals and public officials. The German newspaper Süddeutsche Zeitung, which had obtained the files from an anonymous source, shared them with the International Consortium of Investigative Journalists (ICIJ), and that organization passed them on to a large network of international news outlets, including BBC and the Guardian. According to the so-called Panama Papers (Panama’s government has strongly objected to the name), among the people who used offshore tax havens to shelter their fortunes were the presidents of Argentina, Ecuador and Ukraine; the king of Saudi Arabia; the prime ministers of Australia and Iceland; members of the Spanish royal family; and a number of prominent athletes, actors and businesspeople around the world.

Queen Elizabeth II, U.S. Commerce Secretary Wilbur Ross, Facebook, and Nike were among the prominent corporations and individuals named in the 13.4 million confidential documents that were dubbed the “Paradise Papers.” Stolen from the offshore law firm Appleby, and leaked to the German newspaper Süddeutsche Zeitung (who also received the Panama Papers in 2016), these documents revealed around $10 trillion in offshore investments that were used to hide wealth and profit and avoid taxes.