Large conspiracies can't stay secret for long. People are not reliable at keeping secrets.
Yes, they can, actually.
Of course, showing that pretty much needs some being revealed after a good period where they weren't.
Example of one: in the 1970's, Indonesia invaded neighbor East Timoor, using US-supplied weapons that under US law could only be used for defense. 250,000 were killed.
Now, it so happens, they did this one day after President Ford and Henry Kissinger left from a visit; they always claimed that that was a coincidence and Indonesia hid it from them.
Decades later, documents were finally found - thanks, Freedom of Information Act - showing that Ford had approved this illegal violence, and just said to wait until they leave.
This in exchange for things he wanted. 250,000 innocent people traded.
For another example, the most effective UN Secretary General in history was killed in a plane crash said to be an accident. That story held for decades, and it was a major incident - and never was revealed because people can't keep a secret. But science decades later showed the plane had been shot down, IIRC.
For an example of a conspiracy which still hasn't really gotten much public attention, Richare Nixon in 1968, running for President, sabotaged the President's peace talks with Vietnam's two sides because peace would hurt him in the election (and would have prevented it, it turns out). There's some reasonable evidence this was done and how, promising the South Vietnamese things if they rejected any agreement when Nixon took power, but since it's not the 'found a tape recording' type smoking gun, most Americans still don't appreciate the treaon that happened. Speaking of which, Watergate was a conspiracy that held up reasonably well as far as 'people keeping secrets'; it was only people being caught, with a white house contact on a paper on them, that helped break the story - and even then most of the press ignored it; and if Nixon hadn't recorded things such as plotting to pay hush money, with those tapes exposed, many thing Watergate would still be no more than 'rumors' or wrongdoing at high levels, with no more than a low-level aide at fault.
And then there's a conspiracy that Watergate itself was a setup - sabotaged intentionally either as jockying within the administration, or even to bring down Nixon, no one's 'talked'.
Speaking of setups, there's some evidence - not a smoking gun, not a confession fromperpetrators- that the 'security state' intentionally sabotaged the U-2 flight of Gary Power in 1959 in order to prevent the 'peace summit' between the US and USSR, which the incident did. True or not - there was a powerful force in the US government in the 'security state' - recall Eisehower's warning; if you want something more eye-opening, check out a book by the man who was the Pentagon's liason from the Pentagon to the CIA for years, Fletcher Prouty, called 'the secret team'. This guy is a leading figure in 'conspiracy theories' - yet also in a credible position to discuss the history.
(Of course, the U-2 flights themselves were a 'conspiracy theory' the US said the USSR was lying about - until the USSR unexpectedly produced prisoner Gary Powers).
Things we do have good evidence for, for example, are that the CIA treasonously manipulated President Kennedy into the Bay of Pigs, with false assurances the operation would work without US involvement, attempting to get him to approve it expecting that once it started, he couldn't let it fail and would have to approve a US invasion they wanted.
He didn't, and the two top CIA officials found themselves out of work.
(On the same topic, I've read from one of the Cuban leaders of the invasion that they had a secret agreement with the CIA handlers, that if JFK had cancelled the invasion, they'd fake a 'takeover' by the Cubans where they tied up the CIA handlers, and launched the invasion anyway - again treasonous behavior by the CIA).
We know that Kennedy at the time felt that a coup by this security state was possible - and he encouraged, as a warning to the American people, for the book "Seven Days in May" showing a story of that happening to be made into a movie. (The Pentagon resisted the film and refused co-operation; Kennedy arranged for the filmmakers to use the White House for filming).
How about this for a conspiracy theory: When Kennedy took office, the military, under Curtis LeMay, considered the nuclear war plans - which turned out to be a hair-triegger that if there was conventional war between East and West in Europe, where it was very possible over Berlin, then the US would launch an attack on thousands of cities in the USSR and China (China 'just because since we were already attacking') - to be 'theirs' so much that when the Secretary of Defense McNamara said he wanted to see the plans to evaluate them, they refused his request - the plans were none of his business. Sound a bit wild? I heard the story from Robert McNamara personally (and can point you to an online recording of him telling it, if you want).
(In fact, while we're on the military culture in the Kennedy era, at the time each branch had its own intelligence operation, which serve that branch's interests, not the President's. To address this, Kennedy created a new agency, the DIA (Defense Intelligence Agency), combining the operations under one agency - headed by a general loyal to Kennedy.
The interesting history of that can be read in a book written by the son of the man who Kennedy selected to head the new agency, James Carroll, in "House of War".)
Of course, all kinds of conspiracy theories are false, all kinds are 'crazy', all kinds are substantiated; some are also 'planted' intended to discredit other theories.
If you'd like some reading to get a better idea on some of this, I can point you to some suggested reading for a taste.
For example, "Mary's Mosaic", about the murder of JFK's lover - who was also the former wife of a high CIA official, and on a 'mission of peace' and interested in exposing the CIA.
Complete with a 'secret diary' from her conversations with JFK, that mysteriously disappeared after she was killed - her former husband having gone after it.
(Speaking of 'conspiracy theories', consider she was one of a group of Washington peace-loving women who wanted to use LSD given to US leaders to help bring peace.)
I had another recommendation, but it slipped my mind while writing - if you ask, I'll try to remember more.
Anyway, it's a complicated issue - but you can't rely on 'people talking'.
Another example - by the time Daniel Ellsberg leaked the Pentagon papers,
the US had lied to the American people about Vietnam for years - and they'd all 'kept the secret' until this one person decided, risking life in prison for doing so, the first civilian to have seen the document, to expose the lies to the American people, years later. That's a long time to hide awfully big lies by a lot of people.
Many of the biggest conspiracies that have been exposed are really stories of a lot of people keeping secrets very well, with one exception or a freak accident revealing it.
One last 'conspiracy theory' for you to explain - Johnny Roselli was a central figure close to mob leaders and who had worked with the CIA for years on joint projects. In the late 1970's, Congress re-opened the JFK assassination investigation and subpoenad Roselli to testify. The day before his testimony, he was killed gangland style, reportedly with shots around his mouth as a mob message 'don't talk'. Someone cared enough about his talking about that assassination to kill this central figure this way, the timing linking it to that issue.
While we're on that issue, explain this:
At the height of Watergate, Nixon, to save his presidency, desparately wanted to prevent the FBI - who was no longer loyal to him so that he could tell them not to investigate - from investigating the break-in, and the way he could is if the CIA would tell them to do so for national security reasons.
The CIA was resistant to abuse that power for political coruption. So Nixon wanted to pressure them very badly. What did he use to apply pressure?
He sent his aide with a message - the aide didn't have to understand it - to the director of the CIA saying 'if you don't do this, the whole Bay of Pigs thing can come out'.
Now, we know courtesy of Nixon's loyal Chief of Staff, H. R. Haldemann, that 'Bay of Pigs' was a code phrase Nixon used for the JFK assassination; and that the CIA Directory 'erupted in fury' when given this message and stormedout. So, what was that all about?
Edit: while we're on Nixon, JFK and Cuba, it's hard not to add a bit more history with another story: Nixon had false documents created that he claimed to 'find' in the White House that made it look like the Kennedys had done things that were wrong that they had not done. He released them to the press. But, the press determined that they were fakes.
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