The Three Great Powers consisting of the Marine Headquarters, the Seven Warlords of the Sea, and the Four Emperors were the three greatest military forces on the oceans. The balance between them is what maintained world peace. They were first hinted to by Yosaku when explaining how Jinbe unleashed Arlong in the East Blue to Luffy and Sanji.
The Three Great Powers originally consisted of the Four Emperors (四皇 Yonkō?), the four most powerful Pirate Captains in the entire world and their respective crews; the Seven Warlords of the Sea (王下七武海 Ōka Shichibukai?), seven incredibly powerful Pirate Captain that agreed to become allies of the World Government in exchange for a pardon for past and future crimes; and the Marines (海軍 Kaigun?), more specifically Marine Headquarters (海軍本部 Kaigun Honbu?), where the most combat capable Marines in the world, such as the Admirals, are stationed. With the Seven Warlords working together with the Marines, the government was able to maintain a balance of power with the Emperors.
The Five Elders fear that if this balance gets disrupted, the world itself could descend into anarchy. Furthermore, the balance of power was incredibly delicate; if even one party from one these groups is lost, it could make the balance fall apart entirely. After Crocodile was arrested and got his Warlord title revoked following the Arabasta incident, the loss of such a powerful ally in such a public manner tips the balance away from the Seven Warlords of the Sea, and in turn, the Marines as well. This leaves them with diminished force to counter the menace of the Four Emperors and allows the general populace to question their power and authority.
Another concern is that the Marines are the sole organization completely under the supremacy of the World Government. The Seven Warlords often only follow orders on a whim, if at all, and the Four Emperors are outside of the World Government's control entirely.
The Four Emperors and Marines seem to be on the opposite ends of the spectrum, with the Marines being more orderly compared to the Four Emperors who are very chaotic, with the Seven Warlords acting as somewhere in between. Considering that these organizations are stated to be a balance and the fact that the loss of Crocodile threatened the balance, it is likely these groups are approximately equal in strength. According to Yosaku, the presence of the Three Great Powers over the Grand Line is what makes it the "Pirates Graveyard".
As the delicate balance of the Three Great Powers is what stabilizes the world, the existence and actions of other powerful groups, such as the Revolutionary Army and the Worst Generation is a significant threat to that balance. With the current dissolution of the Seven Warlords system, it is currently unknown how much the balance of power has been affected within the world. While several civilians express such concern, the World Government had utilized the Special Science Group (特殊科学班 Tokushu Kagaku-han?), or SSG for short, as a means to replace the Seven Warlords, expressing confidence it will restore the delicate balance. However, with two of the Emperors, Kaido and Linlin, allying together for world domination, they express their confidence in toppling the entire World Government and the Three Great Powers' balance.
Summit War Saga
The arrest of Portgas D. Ace, the second division commander of the Whitebeard Pirates resulted in the Marines entering into hostilities with the Whitebeard Pirates. This eventually led to a major battle between the Marines and Seven Warlords against the Whitebeard Pirates and their allies. This was the first known instance where any of the powers went into full-out conflict with one another. Prior to the battle, Kaido tried to attack Whitebeard and was intercepted by Shanks, causing a dispute between the Emperors. After Whitebeard's death, the balance collapsed, and the world entered into chaos, contrary to the belief that the great pirate's death would ensure peace.
At the conclusion of the Summit War of Marineford, the Four Emperors have lost one from their ranks (Whitebeard), and the Seven Warlords have lost four; Jinbe and Blackbeard resigned their positions (for different reasons: Jinbe because he opposed the war against Whitebeard and changed alliances to protect Fish-Man Island, and Blackbeard only joined for easier access to Impel Down - his ultimate goal being the liberation of several prisoners on Level 6 to augment the strength of his crew), Crocodile was dismissed from his position after his defeat (although he was succeeded by Blackbeard), and Gecko Moria was deemed "too weak" to retain his status. Furthermore, both Monkey D. Garp and Sengoku resigned after the Summit War of Marineford.
During the timeskip, all three groups underwent changes in membership. In the Marines, Admirals Akainu and Aokiji dueled for the right to become Sengoku's successor. Sakazuki won and has been promoted from Admiral to Fleet Admiral, while Kuzan resigned from his position as Admiral. Of the Seven Warlords of the Sea, Boa Hancock, Bartholomew Kuma, Donquixote Doflamingo, and Dracule Mihawk retained their titles, and Trafalgar Law, Buggy, and Edward Weevil were appointed to fill the three vacant positions. Of the Four Emperors, Marco took command of the Whitebeard Pirates, but is not known to have taken his former captain's title of Emperor. A year after Whitebeard's death, the Whitebeard Pirates and Blackbeard Pirates clashed in the Payback War. The Blackbeard Pirates ultimately emerged victorious, and the Whitebeard Pirates have not been seen since. This event, combined with the Blackbeard Pirates conquering the majority of Whitebeard's former territories, led to the World Government acknowledging Blackbeard as a member of the Four Emperors.
After the time skip, Law's status as a Warlord was ultimately revoked due to his forming an alliance with the Straw Hat Pirates, and Doflamingo also lost his status after the Dressrosa incident and the discovery of his abuse of power.
Four Emperors Saga
Monkey D. Luffy infiltrated Whole Cake Island in order to retrieve his chef, where he allied himself with the Fire Tank Pirates, in an attempt to assassinate Big Mom, which ended in failure. However both crews managed to escape her territory, with Luffy's actions indirectly destroying her base.
During the Levely, Kings Nefertari Cobra and Riku Doldo III brought up the facts that their respective countries suffered because of the Seven Warlords of the Sea, which led to the heated debate and ultimate passing of the notion of dissolving the Seven Warlords organization. As part of the action, large fleets of Marines were sent to arrest Buggy, Dracule Mihawk, Edward Weevil, and Boa Hancock.
The end of the Seven Warlords system led to a majority of the world cheering, with civilians believing that allying with pirates was a grave mistake to begin with, and many pirates being fed up with the arrogance of the group. A few, however, became worried at the resulting imbalance of the Three Great Powers. The Marines hope the SSG will successfully replace the Seven Warlords as part of the Three Great Powers.
Big Mom entered Kaido's territory, Wano Country, in pursuit of Luffy. When the two Emperors came face-to-face, they started fighting each other, with the World Government hoping they would inhibit themselves. However, much to their shock, the two ended up allying with each other. It was then revealed during the Fire Festival, that they were launching New Onigashima Project, where they would enslave the remaining populace of Wano, turning the island into a pirate paradise, before aiming for the Ancient Weapons and the One Piece and ultimately initiating the "Final War". However, just after Kaido killed his longtime ally, Kurozumi Orochi and obtaining the former shogun's forces, the Beasts Pirates headquarters of Onigashima came under attack by the Ninja-Pirate-Mink-Samurai Alliance.
- One Piece Manga and Anime — Vol. 45 Chapter 432 (p. 7) and Episode 314, Garp explains the Three Great Powers.
- One Piece Manga and Anime — Vol. 8 Chapter 69 and Episode 31.
- One Piece Manga and Anime — Vol. 95 Chapter 957 (p. 3) and Episode 958.
- One Piece Manga and Anime — Vol. 66 Chapter 650 and Episode 570.
- One Piece Manga and Anime — Vol. 67 Chapter 659 and Episode 583.
- One Piece Manga and Anime — Vol. 70 Chapter 700 and Episode 629.
- One Piece Manga and Anime — Vol. 80 Chapter 802 and Episode 751.
- One Piece Manga and Anime — Vol. 82 Chapter 821 and Episode 773.
- One Piece Manga and Anime — Vol. 72 Chapter 713 and Episode 643.
- One Piece Manga and Anime — Vol. 79 Chapter 792 and Episode 735.
- One Piece Manga and Anime — Vol. 80 Chapter 801 and Episode 746.
- One Piece Manga and Anime — Vol. 95 Chapter 956 (p. 13-17) and Episode 957, The Seven Warlords of the Sea are officially disbanded.