If you're looking for the most valuable forgery/con in history, a good candidate is the Donation of Constantine[1], a forged document which claimed to transfer sovereignty over western europe to the pope.
Another good candidate: the Privilegium Maius [0], which contained several provisions to elevate the status of the House of Habsburg and their territories in central Europe and helped them rise to the first tier of European aristocrat families, where they remained for 500+ years (interestingly, despite the forgery being recognized as such almost immediately).
I've got an old map of Massachusetts from 1799 which, while not fraudulent, is completely wrong in several places. For one, where my town is supposed to be, it names another town that is actually much closer to Boston (and is also duplicately labelled on the map in its correct spot). It isn't like my town didn't exist at that point. It incorporated 50 years before that, and already had a major road from Concord going through it.
It probably started as groups of people trying to survive bandits in a village, then in a castle, then a city state and then on some land. Before you know, there's a different culture.
If you claim sovereignty over a state's territory, violate its laws, refuse to pay its taxes, and threaten its citizens will violence, will the state respond?
If their response is severe enough to bring you into line, the state exists. If not, then the state is fiction.
> He was so convincing that 250 poor Scots went on board the ship, the Edinburgh Castle, armed with his guidebook to Poyai. They then spent two years dying in the jungle. Forty of them come back and stand for his defense to say, ‘No, we just got lost because obviously we didn’t find the land in the guidebook.
[1] https://en.wikipedia.org/wiki/Donation_of_Constantine