“Secrets ... Can Be Kept Reliably ... For Decades … Even Though They Are Known to THOUSANDS of Insiders”

Judges, prosecutors and journalists all know that collusion happens every day....

Bernie Madoff's Ponzi scheme was a conspiracy. The heads of Enron were found guilty of conspiracy, as was the head of Adelphia. Numerous lower-level government officials have been found guilty of conspiracy. See this, this, this, this and this.
Time Magazine's financial columnist Justin Fox writes:

Some financial market conspiracies are real ...
Most good investigative reporters are conspiracy theorists, by the way.

Indeed, conspiracies are so common that judges are trained to look at conspiracy allegations as just another legal claim to be disproven or proven by the evidence.

Federal and all 50 state's codes include specific statutes addressing conspiracy, and providing the punishment for people who commit conspiracies.
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Conspiracy is a very well-recognized crime in American law, taught to every first-year law school student as part of their basic curriculum. Telling a judge that someone has a "conspiracy theory" would be like telling him that someone is claiming that he trespassed on their property, or committed assault, or stole his car. It is a fundamental legal concept.
Obviously, many conspiracy allegations are false (if you see a judge at a dinner party, ask him to tell you some of the crazy conspiracy allegations which were made in his court). Obviously, people will either win or lose in court depending on whether or not they can prove their claim with the available evidence. But not all allegations of trespass, assault, or theft are true, either.
Proving a claim of conspiracy is no different from proving any other legal claim, and the mere label "conspiracy" is taken no less seriously by judges.

Indeed, even Obama's information tzar - who advocates using the power of the state to shut down talk of conspiracies - admits:

Some conspiracy theories, under our definition, have turned out to be true ... The Watergate hotel room used by Democratic National Committee was, in fact, bugged by Republican officials, operating at the behest of the White House. In the 1950s, the CIA did, in fact, administer LSD and related drugs under Project MKULTRA, in an effort to investigate the possibility of 'mind control.'” [This post isn't going down the rabbit hole of mind control.]

Can't Hide It

Those accusing Goldman Sachs, Dick Cheney or some other powerful people of conspiring to enrich their interests are often met with the argument that "someone would have spilled the beans" if there had really been a conspiracy.
But famed whistleblower Daniel Ellsberg explains:

It is a commonplace that "you can't keep secrets in Washington" or "in a democracy, no matter how sensitive the secret, you're likely to read it the next day in the New York Times." These truisms are flatly false. They are in fact cover stories, ways of flattering and misleading journalists and their readers, part of the process of keeping secrets well. Of course eventually many secrets do get out that wouldn't in a fully totalitarian society. But the fact is that the overwhelming majority of secrets do not leak to the American public. This is true even when the information withheld is well known to an enemy and when it is clearly essential to the functioning of the congressional war power and to any democratic control of foreign policy. The reality unknown to the public and to most members of Congress and the press is that secrets that would be of the greatest import to many of them can be kept from them reliably for decades by the executive branch, even though they are known to thousands of insiders.

History proves Ellsberg right. For example:

• One hundred and thirty thousand (130,000) people from the U.S., UK and Canada worked on the Manhattan Project. But it was kept secret for years

• A BBC documentary shows that:

There was "a planned coup in the USA in 1933 by a group of right-wing American businessmen . . . . The coup was aimed at toppling President Franklin D Roosevelt with the help of half-a-million war veterans. The plotters, who were alleged to involve some of the most famous families in America, (owners of Heinz, Birds Eye, Goodtea, Maxwell Hse & George Bush’s Grandfather, Prescott) believed that their country should adopt the policies of Hitler and Mussolini to beat the great depression"

Moreover, "the tycoons told General Butler the American people would accept the new government because they controlled all the newspapers." Have you ever heard of this conspiracy before? It was certainly a very large one. And if the conspirators controlled the newspapers then, how much worse is it today with media consolidation?

• 7 out of the 8 giant, money center banks went bankrupt in the 1980's during the "Latin American Crisis", and the government's response was to cover up their insolvency. That's a cover up lasting several decades

• Governments have been covering up nuclear meltdowns for fifty years to protect the nuclear industry. Governments have colluded to cover up the severity of numerous other environmental accidents. For many years, Texas officials intentionally under-reported the amount of radiation in drinking water to avoid having to report violations

• The government's spying on Americans began before 9/11 (confirmed here and here. And see this.) But the public didn't learn about it until many years later. Indeed, the the New York Times delayed the story so that it would not affect the outcome of the 2004 presidential election

• The decision to launch the Iraq war was made before 9/11. Indeed, former CIA director George Tenet said that the White House wanted to invade Iraq long before 9/11, and inserted "crap" in its justifications for invading Iraq. Former Treasury Secretary Paul O'Neill - who sat on the National Security Council - also says that Bush planned the Iraq war before 9/11. And top British officials say that the U.S. discussed Iraq regime change one month after Bush took office. Dick Cheney apparently even made Iraqi's oil fields a national security priority before 9/11. And it has now been shown that a handful of people were responsible for willfully ignoring the evidence that Iraq lacked weapons of mass destruction. These facts have only been publicly disclosed recently

These examples show that it is possible to keep conspiracies secret for a long time, without anyone "spilling the beans".

Discipline

To anyone who knows how covert military operations work, it is obvious that segmentation on a "need-to-know basis", along with deference to command hierarchy, means that a couple of top dogs can call the shots and most people helping won't even know the big picture.

Moreover, those who think that co-conspirators will brag about their deeds are forget that people in the military or intelligence or who have huge sums of money on the line can be very disciplined. They are not likely to go to the bar and spill the beans like a down-on-their-luck, second-rate alcoholic robber might do.

Finally, people who carry out covert operations may do so for ideological reasons -- believing that the "ends justify the means". Never underestimate the conviction of an idealogue.

"All Wars Are Based on Propaganda"
Tom Brokaw notes:

All wars are based on propaganda.

There are numerous examples, such as dressing corpses in fake uniforms and planting false intelligence on them to mislead enemies.
In addition, governments from around the world have admitted to false flag terror. In all of these cases, more than a handful of people were involved in planning, funding and carrying out the attacks. And yet no one spilled the beans or – if someone did – they were not believed.

For example:

• The CIA admits that it hired Iranians in the 1950's to pose as Communists and stage bombings in Iran in order to turn the country against its democratically-elected prime minister

• Israel admits that an Israeli terrorist cell operating in Egypt planted bombs in several buildings, including U.S. diplomatic facilities, then left behind "evidence" implicating the Arabs as the culprits (one of the bombs detonated prematurely, allowing the Egyptians to identify the bombers, and several of the Israelis later confessed) (and see this and this)

• The former Italian Prime Minister, an Italian judge, and the former head of Italian counterintelligence admit that NATO, with the help of the Pentagon and CIA, carried out terror bombings in Italy and other European countries in the 1950s and blamed the communists, in order to rally people’s support for their governments in Europe in their fight against communism. As one participant in this formerly-secret program stated: "You had to attack civilians, people, women, children, innocent people, unknown people far removed from any political game. The reason was quite simple. They were supposed to force these people, the Italian public, to turn to the state to ask for greater security" (and see this)(Italy and other European countries subject to the terror campaign had joined NATO before the bombings occurred)

• As admitted by the U.S. government, recently declassified documents show that in the 1960's, the American Joint Chiefs of Staff signed off on a plan to blow up AMERICAN airplanes (using an elaborate plan involving the switching of airplanes), and also to commit terrorist acts on American soil, and then to blame it on the Cubans in order to justify an invasion of Cuba. See the following ABC news report; the official documents; and watch this interview with the former Washington Investigative Producer for ABC's World News Tonight with Peter Jennings. Official State Department documents show that - only nine months before - the head of the Joint Chiefs of Staff and other high-level officials discussed blowing up a consulate in the Dominican Republic in order to justify an invasion of that country. (While the Joint Chiefs of Staff pushed as a serious proposal for Operation Northwoods to be carried out, cooler heads fortunately prevailed; President Kennedy or his Secretary of Defense Robert McNamara apparently vetoed the plan)

• The South African Truth and Reconciliation Council found that, in 1989, the Civil Cooperation Bureau (a covert branch of the South African Defense Force) approached an explosives expert and asked him "to participate in an operation aimed at discrediting the ANC [the African National Congress] by bombing the police vehicle of the investigating officer into the murder incident", thus framing the ANC for the bombing

• An Algerian diplomat and several officers in the Algerian army admit that, in the 1990s, the Algerian army frequently massacred Algerian civilians and then blamed Islamic militants for the killings (and see this video; and Agence France-Presse, 9/27/2002, French Court Dismisses Algerian Defamation Suit Against Author)

• Former chairman of the Joint Chiefs of Staff General Hugh Shelton says that a Clinton cabinet member proposed letting Saddam kill an American pilot as a pretext for war in Iraq (and see this)
• According to the Washington Post, Indonesian police admit that the Indonesian military killed American teachers in Papua in 2002 and blamed the murders on a Papuan separatist group in order to get that group listed as a terrorist organization.

• The well-respected former Indonesian president also admits that the government probably had a role in the Bali bombings

• As reported by BBC, the New York Times, and Associated Press, Macedonian officials admit that the government murdered 7 innocent immigrants in cold blood and pretended that they were Al Qaeda soldiers attempting to assassinate Macedonian police, in order to join the "war on terror".

• Former Department of Justice lawyer John Yoo suggested in 2005 that the US should go on the offensive against al-Qaeda, having “our intelligence agencies create a false terrorist organization. It could have its own websites, recruitment centers, training camps, and fundraising operations. It could launch fake terrorist operations and claim credit for real terrorist strikes, helping to sow confusion within al-Qaeda’s ranks, causing operatives to doubt others’ identities and to question the validity of communications.”

• United Press International reported in June 2005:

  U.S. intelligence officers are reporting that some of the insurgents in Iraq are using recent-model Beretta 92 pistols, but the pistols seem to have had their serial numbers erased. The numbers do not appear to have been physically removed; the pistols seem to have come off a production line without any serial numbers. Analysts suggest the lack of serial numbers indicates that the weapons were intended for intelligence operations or terrorist cells with substantial government backing. Analysts speculate that these guns are probably from either Mossad or the CIA. Analysts speculate that agent provocateurs may be using the untraceable weapons even as U.S. authorities use insurgent attacks against civilians as evidence of the illegitimacy of the resistance.

• Quebec police admitted that, in 2007, thugs carrying rocks to a peaceful protest were actually undercover Quebec police officers

• At the G20 protests in London in 2009, a British member of parliament saw plain clothes police officers attempting to incite the crowd to violence

• There are many other instances of false flag attacks used throughout history proven by the historical evidence. See this, this and this.

In summary, the claim that conspiracies couldn't have happened or else we would have known when someone bragged about their deeds completely false.

Conspiracies are generally not discovered unless the facts are aggressively investigated and prosecuted....