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This doesn't necessarily mean it's not true, but in certain cases, like the fall of the Roman empire, the evidence is probably kind of weak, unless your alien friends have let you in on the existence of a CIA time machine.The evidence for covert operations is by definition weak.
The historical precedent though of other acts (that have been verified or been unclassified) in the past 60 years, speaks volumes...
Let's put it this way: countries with global reach, and interests in controlling the whole world, especially areas with natural resources, trade ways and such, are not that many. Even less have worldwide operations, tons of equipment and expertise, the most well oiled armies/agencies in the world, and a historical penchant for messing outside their borders (e.g. http://en.wikipedia.org/wiki/Mohammad_Mosaddegh )and crude propaganda (e.g. http://en.wikipedia.org/wiki/Operation_Mockingbird ). And even fewer believe in things like "Manifest Destiny" or that they are the model country for the world (hardly any European country believes such crap, besides maybe Germany, and that didn't turn out that well for them).
I don't think NK is one of them. It might qualify for some stuff (dellusion, crude propaganda, but then again noone outside their borders believes it), but it hardly qualifies for others...