Three Great Powers

The Three Great Powers (三大勢力, San Dai Seiryoku), composed of the Navy Headquarters, the Seven Warlords of the Sea, and the Four Emperors, are the three greatest military forces on the oceans, and their relations are very vital to the World Government. The three powers exist in a delicate balance of power, not all the details of which are known. It is however known that only the other two organizations are equivalent to the Four Emperors.

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 * —About the Three Great Powers.

Overview
Considering that these organizations are stated to be a balance and the fact that the loss of Crocodile threatened the balance, it is likely they are approximately equivalent in strength. The Four Emperors and Navy seem to be on the opposite ends of the spectrum, with the Marines being more orderly compared to the Four Emperors which are very chaotic, with the Seven Warlords of the Sea acting as somewhere in between.

The Five Elder Stars fear that if this balance gets disrupted, the world itself could descend into anarchy, and the balance itself appears to be very fragile as the loss of even one key member (such as a Warlord) can disrupt the balance. This is why the defeat of Crocodile was so distressing to them; 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 Navy. 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 of the Sea often follow orders on a whim, if at all, and the Four Emperors are entirely outside of the World Government's control.

As the delicate balance of the Three Great Powers is what stabilizes the world, the existence and actions of the Revolutionary Army is a threat to that balance.

Synopsis
The arrest of Portgaz D. Ace, the second division commander of the Whitebeard Pirates resulted in the Navy entering into hostilities with the Whitebeard Pirates. This eventually led to a major battle between the Navy and Seven Warlords of the Sea 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 Four 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 Battle of Navyford, the Four Emperors have lost one from their ranks (Whitebeard), and the Seven Warlords of the Sea have lost four; Jimbei and Blackbeard resigned their positions (for different reasons: Jimbei because he opposed the war against Whitebeard and changed alliances to protect Merman 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 Battle of Navyford, causing a duel between two Admirals for the right to become Sengoku's successor. The duel itself resulted in the resignation of Aokiji, yet another major player in the Navy.

After the timeskip, their status are not fully known aside from a new Fleet Admiral (Sakazuki), the resignation of Kuzan from his rank as Admiral, and Boa Hancock, Bartholomew Kuma, Don Quixote Doflamingo, and Dracule Mihawk still retaining their titles. It has also been stated that Blackbeard has conquered most of the territory Whitebeard left behind and has since accroached the position of Four Emperors. After the timeskip, Trafalgar Law, Buggy and Edward Weevil have been appointed to the vacant positions of Warlord. However, Law's status has been revoked due to forming an alliance with the Straw Hat Pirates, and Doflamingo also lost his status after the Dressrosa incident and the discovery of his power abuse. Among the Four Emperors, Blackbeard was revealed to have conquered the majority of Whitebeard's former territories, taking over as the newest Emperor.