There was a time when nations were so soundly defeated by their opponents that they only left the echos of their existence ring on through fragmented 2nd and 3rd hand (suspicious) accounts and maybe material proof of their existence.. This isn't a "cover-up" in the *intentional* sense, but it's being covered up none the less.
What once was facts and knowledge can be distorted and misreported as time passes on, eventually turning into something else entirely.
Countries today may have committed actions that would never be discovered, either by lack of interest, or a distant enough trail that most wouldn't or couldn't connect them to the incident. This would certainly be considered part of "history", especially depending on the circumstances.
The question isn't if these things have happened, it's deciding whether or not one would count them as a "cover up" in some sort of conspiratorial sense, or simply covered up by the same ever-moving force that will eventually consume us all. Time.
Can a country's history be covered up? It is certainly possible, given the proper circumstances. If a country in the distant past was annihilated by another country, the conquering country could make a concerted effort to purge all records of the conquered country's existence. Of course this would be a serious challenge, given that other countries could have reference or records of that country's existence, but it is theoretically possible.
On a more modern level, a totalitarian country can attempt to cover up its own history. This has been most notably observed in the former Soviet Union and to some degree in North Korea. However these attempts have proven to be only superficially successful, as it seems the majority of the citizens have known on some level that the coverup was a lie.
I'm canadian and I would like to say in bold capital letters: WE BEAT YOU, WE TINY CANADIANS WITH OUR TINY BOWS AND ARROWS MANAGED TO BEAT THE BIG AMERICANS AND BURNED DOWN YOUR CARPING CAPTAL AND ALL WE GET IS ONE LINE IN YOUR HISTORY TEXTBOOK? Sorry to offend, continue.
E.g. Germany is one of the responsible ones for both World Wars, yet as 100th anniversary of the WWI is getting closer they are trying to change history and blame Serbia for starting a war. They do this using lobbies, who are much more interested in money than in truth. Lobbies control media and it's safe to assume that a lot of people will believe in their lies due to poor knowledge and ignorance.
I can't make you accept my opinion on anything, but I can ask you to whenever you read or hear something do it carefully and with caution, because nothing is more destructible than a delusion.
Take Ireland for instance. They have nothing to deny because they have a clean and peaceful history. Lol. Denmark on the other hand has a bad rep for historical reasons (like cheating Greenland). They have nothing to deny. It just would not happen lol. Whether they are proud of their history or not is another story, but there is nothing to deny.
So that idea is extremely dumb.
There is a theory that can be summed up as "the history is told by the winners" - in a sense of the side that won a war, or an everlasting government that stands for decades etc.
This way, it is possible that history can be somehow distorted in order to picture facts in a partial fashion.
Anyway, I do not believe that "covering up" is completely possible, specially regarding a country's history.
Adam2 is a revisionist nazi and a liar who wants people to believe the Danish people violently subjugated and oppressed the landmass of Greenland.
He has no proof of this, as the Danes inherited the colony via a royal marriage, but that does not stop him from broadcasting hist anti-Scandinavian lies and hatred.