Let’s summarize things first :
As we know, there was an age called “The Void Century” in OP’s world timeline. Except Roger’s pirate, not a single living soul on this planet have any idea what the really happened within that gap.
However an Oharan archelogoist, Profesor Clover, had a slight hint about it. He told us that in that time there was once stood a prosperous and powerful ancient kingdom that ruled over the ocean. However, They had a certain enemy. This enemy felt threaten by the kingdoms existence and their certain ‘policy’. So then they invaded the kingdom and succeded. The kingdom was fallen to shred and somehow, the enemy managed to wipe out every single information about them from history.
Coincidentally, The end of the kingdom’s reign also marked the end of The Void Century. At the same time, an organization called The World Government was built for the first time. Clover had a theory that the WG was none other than the enemy itself.
Now let’s enter the theory :