Although there are events that many/most Americans may not be aware of, there isn't really anything hidden like Tiananmen square is hidden.
Seeing as we're on the internet, that's a good first point to make: the internet in the US is uncensored. Even if as an American I wasn't supposed to hear about The War of 1812 (or whatever), there is no mechanism to stop me reading about it on some internet message board. As such, keeping anything hidden in the modern age is pretty impossible without widespread internet censoring, which just doesn't exist in the US.
There are many events in international history that the US were either directly or indirectly involved in. They aren't hidden per se, but ignored or glossed over. There are plenty of horrific events that the US (via the CIA) supported in the name of the Cold War. One thing that comes to mind is the widespread massacre of Communists in 1960s Indonesia.
As I've spent the last hour deleting terrible posts, consider this a reminder. Remember the rules. As was explained in the excellent recent meta post on "What it means to post a good answer in /r/AskHistorians": If you're choosing to answer a question in /r/AskHistorians, there are three questions you should ask yourself first in turn:
Do I, personally, actually know a lot about the subject at hand?
Am I essentially certain that what I know about it is true?
Am I prepared to go into real detail about this?
And also:
Do not post hopelessly incomplete answers based on a skimming of a Wikipedia article just because nobody has yet replied after a few hours.
Sorry, we don't allow throughout history questions. These tend to produce threads which are collections of trivia, not the in-depth discussions about a particular topic we're looking for. If you have a specific question about a historical event or period or person, please feel free to re-compose your question and submit it again. Alternatively, you may PM /u/caffarelli to have your question considered for an upcoming Tuesday Trivia thread.
I am not sure if its " hidden from Americans" but it certainly doesn't receive attention it deserves is the history of Labor in the United States and the massacres that took place by our gov't to suppress them.
http://en.wikipedia.org/wiki/Memorial_Day_massacre_of_1937
http://en.wikipedia.org/wiki/Ludlow_Massacre
People today don't appreciate how much was given to allow us the simple right to not work on weekends or get holidays.
In Tulsa, Oklahoma on June 21, 1921 a black shoe shinier was accused of sexually assaulting a white woman in an elevator and a local paper eager to win a circulation war published a front-page headline declaring "To Lynch Negro Tonight." Hundreds of blacks from the Greenwood neighborhood of Tulsa descended on the courthouse to protect Rowland (the accused) from being lynched by the nearly 10,000 whites (backed by the white police force) who where waiting outside of the courthouse. Once these two groups met, an all out race war began and it appeared as if the local whites wanted to eradicate any trace of black existence in the city. 1,200 homes in the Greenwood neighborhood were burned and close to 300 people were killed in the violence. It wasn't until the year 2000 that Tulsa and the Oklahoma state legislature came to grips with the past and set up a Race riot Commission to investigate and pay reparations to the survivors. Until that happened, the riot was kept under wraps for 80 years because the local newspapers cleaned out their archives to remove evidence of the incident.
Taken from Kenneth C. Davis's Don't Know Much About History which is a fantastic read for anyone interested in a different look at US History .
I'm no expert on Chinese censorship, and would be very interested in reading a good response from someone with that background. But I can speak to the history of the US government's active manipulation of public opinion regarding UFO reports. And we can see how some of the behavior involved might generalize to censorship in a state where most information is technically free and available.
A great starting point is George Orwell's introduction to Animal Farm:
Unpopular ideas can be silenced, and inconvenient facts kept dark, without the need for any official ban. Anyone who has lived long in a foreign country will know of instances of sensational items of news — things which on their own merits would get the big headlines-being kept right out of the British press, not because the Government intervened but because of a general tacit agreement that ‘it wouldn’t do’ to mention that particular fact.
Now we can compare this to early commentary about the censorship of UFO reports in the US press. As basic factual background, "flying saucer" reports exploded into public consciousness when there were some 800 reports in the US over 30 days in the summer of 1947. There was another much larger wave of reports in 1952, including reports of UFOs seen on radar over Washington DC. This was no trivial matter: the UFOs were tracked by radar, jets were scrambled to intercept them, and there was an official military press conference afterwards. The FBI and the Army Air Force were both early investigators of these reports. The CIA came into existence around the same time as the early reports, and they too were involved with early investigations. The Air Force even had an official program started in 1948 that was eventually called Project Blue Book and whose existence was known to the public.
In early 1953 the CIA convened the now-infamous "Robertson Panel". They got a number of scientific experts, briefed them in secret about UFO reports by USAF intelligence, and then took the panel's conclusions. One of the Panel's recommendations was an education and debunking campaign:
The "debunking" aim would result in reduction in public interest in "flying saucers" which today evokes a strong psychological reaction. This education could be accomplished by mass media such as television, motion pictures, and popular articles. Basis of such education would be actual case histories which had been puzzling at first but later explained. As in the case of conjuring tricks, there is much less stimulation if the "secret" is known. Such a program should tend to reduce the current gullibility of the public and consequently their susceptibility to clever hostile propaganda.
This specific section was redacted in early publicly available versions of the document. Its existence is only known because an accidently unredacted copy was in the files of the Unviersity of Colorado UFO research program in the 60s, and Dr. James McDonald found a copy, was scandalized by the implications of government manipulation in scientific research, and published it. So in a sense this information was "leaked". Interestingly enough, the term "Unidentified flying object" was originally used by FBI and Army Air Force intelligence. It was also "leaked" by Donald Keyhoe, a writer who graduated from the Naval Academy and who had very good contacts in the military and intelligence community. The Oxford English Dictionary even gives Keyhoe credit for first using the phrase in print, despite it being used in intelligence documents as early as 1947. So even the word "UFO" was originally a (very unimportant) state secret!
Now consider the following excerpts from Coral Lorenzen's book The Great Flying Saucer Hoax (1962). She devotes a short chapter to "Flying Saucers and Censorship". The first bit describes a meeting she had with J. Allen Hynek, an astrophysicst who advised the Air Force on flying saucer reports from the 40s until the 60s when he quit and began his own public investigations. Also present was Lt. Robert Olsen from Blue Book.
Then the lietenant dropped the bombshell: "We're going to try to keep such reports out of the papers." I was stunned. How could such a thing be accomplished in a free country with a free press? It would have to involve the wire services, and although this sitionation later existed I realize now that it must have been very subtly accomplished. During the flaps in the summer of 1954 and 1956 sightings were made throught the United States in great numbers, but the wire services did not carry the reports. A man who observed an object in Texas, for example, would have no way of knowing that similar objects were being reported elsewhere in the country.
The next relevant bit goes right back to Orwell:
It is not commonly known that within the American (civilian) UFO research field itself, there exists a type of censorship. It was instituted as early as 1954 when accounts of "little men" and landings began to come out of France and South America. Certain elements in the research field either did not accept the information because it came from "foreigners", or because the reports were too bizarre to believe.
Finally, Lorenzen quotes a journalist at length. Their account of UFO self-censorship was written in 1957:
When the saucers hit us most reports and editors realized immediately, if dimly in a good many cases, some of the implications of the Arnold sighting and what followed. You will recall the incredible amount of space devoted to saucers those first few days. This was because reporters and editors were writing their heads off about the hottest subject of the day -- and because noboy had told them to hold down. Things got pretty much out of hand...So staffs got the word to go easy on saucer copy...There never was any flat order except to exercise due caution, but it became apparent that to get a saucer story printed you had to have very solid evidence and attribution. Later on it got so that you wouldn't even get a saucer story on the wire.
This I think paints the basic picture of UFO censorship in America. There are certain controversial facts, here the reports of unexplainable "objects" apparently maneuvering through US air space with utter disregard for our air defense capabilities. These facts are highly embarassing to the relevant intelligence agencies who are tasked with national security at the height of the Cold War (note that in the 40s and early 50s, it was more common for people to think UFO reports were due to atomic weapons or Russia than aliens or whatever). The CIA decides to start actively manipulating public opinion through the normal channels of the distrubution of information. This chilling of policy was also felt in the USAF, who literally had presented to the Robertson Panel a suggestion that the UFO reports could be interplanetary probes. This chilling percolated through society, with local journalists or even UFO witnesses self-censoring because these sort of ideas simply aren't proper. And in the place of any knowledge of actual reports that occur in reality and are documented scientifically and noting their unique features, culturally the concept of "UFO" has essentially been usurped by a science fiction ET cartoon.
I think there are many subjects like this in America. How many people know that the Republic of Lakotah is actively trying to remove themselves from the United States because the US violated the original treaties granting them Lakotah land? How many people know about Fred Hampton, the Black Panther and civil rights organizer who was murdered by the FBI? These are very signifigant examples of how the US government handles dissent, and yet they aren't talked about in polite intellectual company.
TL,DR
"Unpopular ideas can be silenced, and inconvenient facts kept dark, without the need for any official ban."
The Tuskegee Syphilis Experiments could be a good candidate for an answer. And all the eugenics experiments in American prior to WWII.
http://en.wikipedia.org/wiki/Tuskegee_syphilis_experiment
Others I can think of are the Japanese internment camps, and the extent of the US governments involvement in overthrowing democratically elected governments in Central and South America during the 50's and 60's.
That Thomas Jefferson (third President of the United States and primary author of the Declaration of Independence) kept an enslaved woman named Sally Hemmings as a concubine and bore children with her after his wife died is perhaps fairly well known, but the fact that she was almost certainly his wife's half-sister isn't mentioned nearly enough. That a venerated American 'Founding Father' owned some of his own children and their mother (also his wife's sister) as chattel doesn't paint as rosy a picture of the country's origins as some might prefer, I suppose.