One Piece is a 20-year-long running manga and anime that follows the journey of the aspiring pirate king, Monkey D. Luffy, and his 10-member pirate crew composed of different people from all parts of the world.
If you’re new to watching One Piece, it is established as early as the Romance Dawn Arc that the main ‘villains’ of the story is the government or the Marines. During the early parts of the story, it only made sense because the pirates are supposed to be the bad guys, but because the protagonist of One Piece is a pirate himself, then it’s reasonable that the bad guys in the readers’ or watchers’ perspective should be the Marines.

However, as the story goes on, the ‘World Government’ being the true enemy made much more sense, and that’s not solely because they were chasing pirates. Another Spoiler Alert! From here on out, the discussion would include scenes from the manga (yet-to-be animated chapters) and the anime from Episode 227 up. Read at your own risk.
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Going back, the World Government started showing its true colors when Nico Robin, the Devil’s Child was taken hostage during Water 7. It was her past that revealed what kind of evil actually runs the government. After the raid at Enies Lobby, it was immediately followed by the dark past of those living in Sabaody.
The Mariejois, the one that separates the Old and New World was first revealed in Sabaody. We also find out that slavery is a known practice among the World Nobles or the Celestial Dragons, and that Slave Trade has been going on for a long time.
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Note that the World Nobles are under the World Government, they command most of those sitting at the top because they are one of the elite who helped the world rise back up after the Void Century.
Nothing is made clear about the Void Century, but we do know that the World Government is trying its best to conceal whatever happened before that time. This includes the existence of Joyboy and the three weapons. I will not go into further detail about what Joyboy’s role is, but it is obvious that whatever he did was a big threat to the World Government, so they had to hide it at all costs.

In summary, the World Government has some notorious acts up its sleeves, and they don’t want the world to know about what happened. At this point in time, in both the manga and the anime, they are still the biggest threat and the main antagonistic faction of One Piece.
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