The World Government is one of the strongest organizations in the One Piece world and it was created long ago, when 20 Kings banded together in ancient times to fight an enemy that is believed to have been the Ancient Kingdom. After emerging victorious and clearing all track of the war, the World Government was established and it has, since then, been ruling the entire world, with over 170 countries affiliated with it today.
The World Government is incredibly powerful, however, there are still those who oppose it. As such, it has been attacked on a few occasions.
6 Void Century
The earliest attack on the World Government was most certainly one that was led by none other than the Ancient Kingdom, and possibly the previous Nika, or Joy Boy. It is believed that the 20 Kings banded together in this era to put a stop to the Ancient Kingdom. Why they wanted this Kingdom to be erased is not known just yet, however, it could potentially have been for its vast knowledge and superiority over all the other kingdoms of the world.
Deeming it to be a threat, the World Government attacked the Ancient Kingdom and the latter retaliated. They even built Ancient Weapons in order to fight back against the World Government and most likely did considerable damage to the people who had banded together. Unfortunately, eventually, the Ancient Kingdom was completely wiped out and even the study of the ancient scripts about it was completely banned.
5 Iron Giant
Nearly 200 years ago, there was another documented attack on the World Government. This was made known recently by Dr. Vegapunk in One Piece's Egghead Island arc, where he revealed that in search of a power source, he was eventually led to the Iron Giant, which was powered up by an ancient fuel. According to Dr. Vegapunk, this Iron Giant was created by the people of the Ancient Kingdom and it was somehow working until 200 years ago.
Eventually, it attacked Mariejois and launched a full-scale attack on the World Government. Evidently, the Iron Giant was somehow stopped by the World Government and subdued, however, it isn't known how exactly this was accomplished. Curiously, the Iron Giant attacked the World Government around the same time when the Fishmen were declared to be a separate race for the very first time in the One Piece world. Perhaps, the two events are somehow connected.
4 Rocks Pirates
The Rocks Pirates are known to have been an incredibly powerful bunch that existed nearly 40 years ago in the One Piece world and ruled the seas all alone. The group was led by none other than Rocks D. Xebec, and it consisted of some heavyweights such as Edward Newgate, Charlotte Linlin, Kaido, Shiki, among many others.
Rocks aimed to become the King of the World and, as such, his group often acted as a terrorist organization. They launched large-scale attacks wherever they wanted and didn't even spare the Celestial Dragons. Most notably, they attacked God Valley, where a lot of Celestial Dragons were present. Sadly for them, it is there where this group fell eventually, thanks to the collective effort of the Roger Pirates and the Navy.
3 The Straw Hat Pirates
In the Enies Lobby arc, the Straw Hat Pirates declared war on the World Government as well. This was done simply because Nico Robin, the archeologist of the Straw Hat Pirates was in the grasp of the World Government. She tried to sacrifice herself for the sake of the pirate crew, however, knowing the pirate that Luffy is, he did not stand for this. Instead, Luffy and the entire crew infiltrated the judicial island of the World Government, Enies lobby, and there, declared war on the World Government by burning down their flag.
Ever since then, the group has been in direct rebellion against the World Government and the Celestial Dragons. The conclusion of the events of Enies Lobby led to the destruction of the entire island, which was caused by the Buster Call that was accidentally triggered by Spandam, the then captain of the CP9.
2 Latest Reverie
At the latest Reverie that just took place after the events of the Whole Cake Island arc of One Piece, another attack was declared on the World Government. This one was led by none other than the members of the Revolutionary Army, who are known to be in direct rebellion against the World Government. To the government, Dragon and the Revolutionary Army are the biggest enemies.
Over the years, this organization has slowly been growing in power, and now, they are ready to launch a full-scale attack against those in power. At the Reverie, Sabo, along with three of the five Commanders of the Revolutionary Army infiltrated the Holy Land of Mariejois and manage to sneak into the Pangea Castle. There, they burned the flag of the World Government after confronting two Marine Admirals and freeing Bartholomew Kuma, one of their members.
1 Final Saga
In the final Saga of One Piece, the biggest war will eventually break out and it will be against those of the World Government. This is something that every fan of the series knows already and even the author of the story, Oda, has teased it on various occasions.
According to Oda, this war will be so massive that even Marineford will look rather small in comparison. This war is something that the story has slowly been building towards ever since its very beginning and it will certainly be one that will surpass every other in the story. Most likely, this war will see the World Government finally crumble and a new system be established.