Proposed Canon
The Margarine Empire was a side in Erfworld and presumably Royal. It waged a successful war against Dystokyo, croaking Emperor Scup, capturing the capital, and capturing Emperor Scup's heir, Empress Saltina, though Empress Saltina refused to turn and would eventually escape into the Magic Kingdom.
 Real World References
It may be a reference to the fact that Margarine was developed as the result of a challenge proposed by Emperor Louis Napoleon III.