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The World Government from One Piece may seem to care about the welfare of people, but the majority are power hungry and corrupt. They use the navy to hunt down pirates and anyone who disobey them. They ally with 170 counties and run the marines they use to commit questionable actions such as killing people who they think are too powerful to control. They believe that their word is final, true, and ultimately just. Those who oppose them are villains and enemies of the world itself. Within the World Government are some honest good men with good intentions, however they are heavily outnumbered by the more corrupt and power hungry individuals. Because of this, they are one of the main antagonists of the series.
About one millennium ago, the World Government appeared after the Void Century. As it appeared, it managed to get many powerful families and countries to join them. Over time, the World Government manage to gain the support of 170 countries.
The Pirate King's Execution
Pirate King, Gol D. Roger turned himself in after finding the One Piece and living his dreams. Before the execution, Roger entrusted Monkey D. Garp to take care of his son, Ace, which the latter reluctantly agreed to. After Gol D. Roger's execution, a pirate era emerged much to the World Government's disdain.
The Gorosei was cracking down on the archaeologists on the island of Ohara because they realized that the archaeologists found out about the World Government's past. The Gorosei sent Spandine there to investigate he situation. After discovering Nico Olvia (an infamous archaeologist), Spandine ordered a Buster Call (a destructive attack from the Navy) on the small island. Fortunately, Nico Robin escaped from the incident alive thanks to Aokiji but because of her ties to her mother, Nico Olvia, and knowing a bit about the past of the World Government, she was branded the name "Devil Child" and labeled an international criminal.
East Blue Saga
Monkey D. Luffy earned a 30,000,000 beli bounty from the corrupted marine, Nezumi because Luffy attacked him and defeated Arlong and his comrades. The marines are also notably annoyed at Luffy's efforts in taking down these criminals instead of the marines doing it themselves.
Since Luffy defeated Crocodile in Alabasta, Crocodile lost his position as a Shichibukai and Luffy increased while Roronoa Zoro earned his. Luffy's became 100,000,000 beli while Zoro earned a 60,000,000 beli (for his crimes at Whiskey Peak).
Sky Island Saga
The Gorosei first appeared when they were talking about the recent events that had happened like Crocodile losing the title of Shichibukai and their suspicions about the two Yonko Shanks and Whitebeard. At the same time, Sengoku and the Shichibukai had a meeting about Crocodile's replacement as a Shichibukai in Mariejois.
Water 7 Saga
Aokiji appeared on the Long Ring Long Land Island where the Straw Hats were and decided to test their abilities. After defeating Luffy and leaving, he told Spandam that the Straw Hat crew were heading for Water 7. Nico Robin turned herself in to the Government agents, CP9, because she did not want the Straw Hat crew to suffer because of her. However, she fell into Spandam's trap as he wanted her to help him revive the ancient weapon, Pluton. CP9 also captured Franky because Spandam wanted him to hand over the blueprints for Pluton. But with the efforts of the Straw Hats, the Franky Family, and a few others Robin and Franky were saved. After the Enies Lobby invasion, Luffy's grandfather, Monkey D. Garp was ordered by Sengoku to capture the Straw Hat Pirates but they escaped.
Because the Straw Hat's were responsible for destroying Enies Lobby and defeating CP9 each of them got a bounty and Franky was technically forced to join the Straw Hat Pirates because if not he would have been arrested later. Luffy's bounty now became 300,000,000 beli.
Thriller Bark Saga
The World Government ordered Bartholomew Kuma to tell Gekko Moriah about Marshall D. Teach, the new replacement for Crocodile's position as Shichibukai. Kuma was also ordered by the World Government to capture the Straw Hats on Thriller Bark but Kuma let them go much to frustration of Fleet-Admiral Sengoku.
Summit War Saga
Because Luffy attacked a Celestial Dragon, Admiral Kizaru arrived to Saboady Archipelago to take care of business but Bartholomew Kuma sent each of the Straw Hat members to a different island for their protection. Few days later, Luffy with the help of Boa Hancock infiltrated the World Government's most powerful goal, Impel Down, to save his brother, Portgas D. Ace. But since he could not meet him there, Luffy assembled a powerful group of individuals like Crocodile, Ivankov of New Kama land, and the former Shichibukai Jinbe to break out of the powerful prison to save Ace.
Whitebeard wages war against the Marines, so that he can rescue his son, Portgas D. Ace, from execution. Because of Ace's blood relation to the Pirate King, the World Government wanted the execution of the latter which it succeeded in. Before Whitebeard died to the injuries inflicted by the Blackbeard Pirates, he announced to the World that One Piece truly exists. After the war, Donquixote Doflamingo was instructed by an unknown government agent to eliminate Gekko Moriah because of how weak the latter is for Shichibukai status. But it was later revealed by Doflamingo that Moriah probably got away. Later on, the Gorosei also talks about Blackbeard and his intentions in the New World.
Luffy's bounty increased to 400,000,000 beli because of his recent actions at Thriller Bark, Sabaody Archipelago, Impel Down, and Marineford.
Because of Whitebeard's announcement to the world that there is a One Piece out there, many people became pirates and sought out to find the One Piece treasure. Since Jinbe, Blackbeard, and Moriah lost their positions as Shichibukai, Trafalgar Law, Buggy the Clown, and an unknown individual have filled in their positions. Around the same time, Sengoku stepped down from the role of Fleet-Admiral and Akainu became his successor after defeating Aokiji in a duel (causing the latter to leave the Marines). Because Akainu is the Fleet-Admiral, he now seeks out more justice in the name of the World Government.
Pirate Alliance Saga
The World Government is concerned about Donquixote Doflamingo's position as a Shichibukai and Luffy and Law's involvement in trying to bring him down. Because of this, Akainu sends Admiral Fujitora to investigate the problem. After Doflamingo's defeat by the hands of the Straw Hat and Law Alliance, the world began to feel the repercussions of the Shichibukai's defeat. Akainu and the Gorosei were shocked and upset to hear about Doflamingo's defeat. After arguing with Fujitora about concealing the truth about the World Government's role in making Doflamingo rule Dressrosa, Akainu ordered Fujitora to bring back Law and Luffy's heads.
The World Government plays many roles in the One Piece world. The leaders of the World Government, the Gorosei have absolute say over everything in the One Piece world. It run the Marines, Cipher Pol Agency, and have the allegiance of about 170 nations and many powerful families such as the Donquixote Family. The World Government also appoints the Shichibukai (pirates who works for the World Government) so that they can be used to enforce the power of the World Government. With that, the World Government tracks down anyone who opposes it by having them captured or having bounties placed upon them.
There are also important locations that the World Government has special control over like Enies Lobby: a place where criminals are judged, Impel Down: the great underground prison, and Marineford: the head of the navy. But the World Government's main headquarters is in the Holy Land, Mariejois, that is located on the Red Line.
The World government also places a lot of power in the hand of the World Nobles or the Celestial Dragons. Because the Celestial Dragons were the descendants of the founders of the World Government, the Celestial Dragons are granted a lot of money and can do whatever they want. The World Government knows about the Celestial Dragons but it still allows them to be within the law no matter how cruel the Celestial Dragons can be.
As a powerful and well-trusted organization by many people in the world, it would like to be seen as powerful. Leaders of the World Government would eliminate anyone who is too weak to enforce its power such as secretly hiring Donquixote Doflamingo to assassinate Gekko Moriah after the Marineford war. The World Government also want to be seen as something perfect and would lie to the public to make sure of it. For example, it covered up the fact that dangerous criminals have escaped from Impel Down to the public so that it can maintain its trust with the people. The World Government can be very secretive about its past and would prevent anyone from studying the Poneglyphs (giant stone tablets). And if anyone at least disobeys this rule then they are now one of the primary enemies of the world. For this reason, there is something about the World Government that the Gorosei does not want the world to know about.