800 years ago, an Ancient Alliance was formed by the kings of Twenty Kingdoms. The alliance included the Donquixote Family and the Nefertari Family. During the Void Century, they fought against the Great Kingdom and successfully overthrew it.
Why or how the Twenty Kingdoms overthrew the Ancient Kingdom remains a mystery. After that, they banned research on the Poneglyphs and established their headquarters in Mary Geoise.
Eventually, by 800 years ago the Alliance became the World Government.
Due to this, the Twenty Kings leading the war against the Ancient Kingdom were referred as the “creators” of the world. Nineteen of the founding royal families moved and lived in Mary Geoise (only the Nefertari family remained in their country).
The World Nobles, also known as the Celestial Dragons, are direct descendants of the founding kings who moved to Mary Geoise. As such, these Nobles possess the authority to do as they please, simply due to their heritage.
All over the world, there are over 170 countries that are affiliates of the World Government. The World Government is apparently ruled by the Five Elders who in actuality answer to a ruler known simply as Im whose existence within the World Government is not known to the world at large.
The Commander-in-Chief Kong oversees both the Marines and Cipher Pol as well as the Shichibukai, at least officially, and ranks directly below the Five Elders.
The Marines serve as their primary military organization, headed by the Fleet Admiral.
The Cipher Pol operate behind the scenes, either through sabotage, reconnaissance, spying, espionage and/or assassination; their most powerful enemies are largely pirates and revolutionaries.
Impel Down serves as the World Government’s most impenetrable maximum-security prison.
Collectively serving as the most prominent antagonists in the series, the World Government is largely portrayed as a corrupt entity, particularly through the application of absolute justice, the suppression of research into the Void Century, and the depravity of the aristocratic World Nobles.