Pirate Alliance

A Pirate Alliance is a pact between two or more pirate captains and their respective crews.

Leadership
The captains who formed the alliance are theoretically equal in status, and cannot give each other orders. They may have limited control over each other's crews, but the highest authority still lies with the respective captains.

In a situation where one of the captains gets captured, the other can take charge of their remaining subordinates.

Goals and Benefits
An alliance is usually formed with the purpose of reaching a specific, mutual goal. It may be something that neither crew could accomplish on their own, such as defeating a powerful enemy.

Issues
A common problem with pirate alliances is betrayal; when one captain sees no more use of the other, it results in treason in order to reduce the needs to share glory. Also, due to their mutual goal being the only thing that binds them together, they usually have little to no respect for members of the allied crews, leading to one insulting the other behind their backs or even attacking them over perceived threats.

For Shichibukai, forming an alliance essentially means forfeiting their status, as the World Government does not permit such affiliations with criminals.

Another problem is that an attempt to even establish an alliance between Captains and their respective crews can result in conflict if it fails like when Shiki and his crew tried to forge and alliance with Gol D. Roger and the Roger Pirates which resulted in the Edd War when Roger rejected Shiki's offer of an alliance.

Buggy and Alvida Alliance



 * Status: Active

The Buggy and Alvida Alliance is an alliance formed by the pirate bands known as the Buggy Pirates and the Alvida Pirates (although only its captain remained from them) and led by the leaders of the groups, Captains Buggy and Alvida. They teamed up in order to defeat a common enemy, Monkey D. Luffy, and remain allied even after this failure, to find Captain John's lost treasure. As Buggy has became a Shichibukai during the time-skip, he is forbidden to hold an alliance with Alvida as equals, though no action or knowledge is seen taken against Buggy.

Saruyama Alliance



 * Status: Active

The Saruyama Alliance is an alliance that was formed between three pirate captains, Montblanc Cricket and his surrogate sons "Salvage King" Masira and his brother "Sonar King" Shoujou. It composes of two pirate crews led by the brothers: the Masira Pirates (マシラ海賊団) and Shoujou Pirates (ショウジョウ海賊団). Unlike many other alliances seen, they are extremely loyal to each other to the end, akin to a family. They work together to salvage evidence for the lost Sky Island, in order to prove Cricket's ancestor, Montblanc Noland, was not a liar. Their alliance continued in search for new dreams, and currently looking for the 'Island of Dreams', Nakrowa.

New Fishman and Flying Alliance

 * Status: Disbanded

The New Fishman Pirates and the Flying Pirates, led by Hody Jones and Vander Decken IX formed an alliance in order to overthrow the Ryugu Kingdom. The two conspired to have Hody break into the palace while Decken could capture Shirahoshi and marry her. However, the plan failed, and the alliance did not last very long, with Hody even declaring Decken useless. After Decken sent Noah after Shirahoshi, Hody stabbed him with a trident in order to send Noah down towards Fishman Island, effectively ending their alliance.

Heart and Straw Hat Alliance

 * Status: Active

On Punk Hazard, the Heart Pirates and Straw Hat Pirates, two crews from The Worst Generation, formed an alliance. The respective captains, Trafalgar D. Water Law and Monkey D. Luffy, teamed up with the sole purpose of overthrowing Kaido of the Yonko. This alliance was proposed by Law, who, at the time, was still a Shichibukai.

However, the true reason for the alliance was to destroy the SMILE Factory that belonged to another Shichibukai Donquixote Doflamingo who allied with Kaido; what Law did not inform Luffy at the time was that he intends to kill Doflamingo and destroy the entire Donquixote Pirates to avenge the death of Donquixote Rosinante. The alliance was placed in the newspaper the next day and all around the world people reacted in shock upon learning of it. Kinemon and Momonosuke joined the group hoping to save Kanjuro. During the Dressrosa Arc, Law called off the alliance while facing Doflamingo but after being captured Luffy forced him back into it stating that it was his choice and not Law's, much to Law's irritation, but reluctant acceptance.

As Law is a Shichibukai, he was forbidden from allying with pirates who are not allied with the Government, and as such his title is revoked. Although they have yet to achieve their promised goal of defeating Kaido, the alliance has succeeded Law's desired result of defeating Doflamingo; the results of this alliance were reported at a global scale. This alliance is notable as the two captains of this alliance Monkey D. Luffy & Trafalgar D. Water Law are both carriers of D.

After the events of Dressrosa the alliance is now official. Some victims of birdcage asked Luffy to take them under his wings, but the rubber man refused and the group introduced an underling's vow forming the "Straw Hat Grand Fleet" a huge group of warriors with seven representatives.

Currently the alliance has the following organization:
 * Luffy, Law, Zoro, Usopp, Robin, Franky, Kinemon, and Kanjuro are on Going Luffy-senpai as guest of Barto Club and are currently heading to Zo.
 * The Heart Pirates (except Law) are on Zo waiting for their captain.
 * The Thousand Sunny's group (Sanji, Nami, Chopper, Brook, and Momonosuke) arrived on a mysterious island carrying Caesar Clown as a hostage. They gained information about a strange girl and another samurai.
 * The Tonta Corps returned to Green Bit. The Tontattas are revealed to the citizen of Dressrosa as dwarves (not fairies) and now are in harmony with the humans. They turned Lucy/Luffy and Usoland/Usopp into national heroes.
 * The Beautiful Pirates, the Happo Navy, the XXX Gym Martial Arts Alliance, Hajrudin and the Yonta Maria Grand Fleet are traveling in the world with a piece of Luffy's Vivre Card.

Kid, On Air, and Hawkins Alliance

 * Status: Active

During the Punk Hazard Arc, three captains of The Worst Generation, Eustass Kid, Scratchmen Apoo and Basil Hawkins, met on the Kid Pirates' base to discuss the terms of an alliance for the sole purpose of defeating Shanks. Despite some minor altercations, the three were eventually able to come to terms, and their alliance was reported in the newspaper. After Law and Luffy's victory over Doflamingo, the three pirates were relieved that the two were not going after the same Yonko they were targeting.

Suddenly, Kaido landed on their base and the three found themselves face-to-face with him.

XXX Gym Martial Arts Alliance



 * Status: Active

After the battle on Dressrosa, four of the gladiators who fought in it decided to form an alliance. These were Ideo, Blue Gilly, Abdullah, and Jeet. Ideo was their representative, and the alliance hoped to become subordinates of the Straw Hat Pirates. Although Luffy ultimately rejected, he offered them friendship nevertheless, which they continue to swear their allegiance as the Fourth Division of the Straw Hat Grand Fleet.

One Piece: Pirate Warriors 2
Several alliances are featured in the game One Piece: Pirate Warriors 2 in the game's original story the post-timeskip Straw Hat Pirates (along with Smoker and G-5 Marines due to a bounty being placed on their heads and Jinbe) and Whitebeard Pirates (which includes Whitebeard & Portgas D. Ace who are still alive) form an alliance to combat the Marine's lead by Fleet AdmiralSakazuki & Buggy's Great Pirate Alliance (Buggy, Galdino, Krieg, Wapol, Gekko Moriah, etc.) which had made a deal with the World Government to collect the bounties on the Straw Hats and Smoker's heads, as well as help Luffy find his crewmates (Usopp, Robin, Zoro, & Sanji) where under the effects of the dark fog produced by the Frenzy Dials.

The Straw Hat and Whitebeard Alliance faces many other Alliances such as an alliance between Crocodile, Arlong, and Perona who banded together to avoid joining the Great Pirate Alliance, though Perona decides to join up with the Straw Hat and Whitebeard Alliance. Boa Hancock joins the Alliance to help them sneak into Impel Down to help Luffy and his allies learn more about the Fog Dials. Eventually the Great Pirate Alliance collapses, due to Blackbeard & Moriah's use of the Control and Mad Dial to take control of countless pirates, marines, and government agents resulting in the creation of the Blackbeard Alliance (made of Blackbeard, Moriah, and Enel along with their brainwashed minions).

To combat the Blackbeard Alliance others join the Straw Hat and Whitebeard alliance such as Kuzan, Bartholomew Kuma, Dracule Mihawk (after defecting from the Blackbeard Alliance), Crocodile, Buggy, Galdino, Monkey D. Garp (after the bounty on Smoker and the G-5 marines is retracted by the Government), and Trafalgar D. Water Law. After managing to removing Blackbeard's control over Vista, Usopp, Robin, Zoro, and Sanji they rejoin their crews in taking down the Blackbeard Alliance and try to form a truce with the Marines, though Fleet Admiral Sakazuki's unwillingness to join forces with pirates results in a huge battle between the two, which Blackbeard takes advantage of to take control of Whitebeard, leading to two alternate endings.

In the standard ending, Whitebeard dies and Blackbeard steals the power of the Gura Gura no Mi, leading to a final showdown between the Straw Hat/Whitebeard Alliance & Marines against the Blackbeard Alliance on Punk Hazard, resulting in Blackbeard's defeat. In true ending, Luffy, Jinbe, Marco, Vista, Jozu, and Ace manage to help Whitebeard regain his senses and Blackbeard loses control over the Frenzy Dials and falls under its effects after the fog fused with one of his Black Holes produced by his Yomi Yomi no Mi along with Sakazuki, Kizaru, Moriah, and Enel who are swallowed up and corrupted as well.

Driven mad by the Fog, the Blackbeard Alliance and Marines lead by Sakazuki attack different regions indiscriminately, while the Whitebeard pirates continued fighting to protect their territories while Garp and Smoker gather the remaining uncorrupted Marine forces to protect public order while The Straw Hat Pirates and Law continued fighting the Blackbeard Alliance, eventually discovering that they are being resupplied with Frenzy Dials to increase their numbers, leading them to the Sky Islands where they are confronted by corrupted Kizaru and Enel.

After freeing them from the dial's control, Kizaru and Enel join the Straw Hat and Whitebeard Alliance in the final battle on Punk Hazard against the corrupted Blackbeard, Moriah, and Sakazuki. The Blackbeard Alliance falls with the defeat of Blackbeard by Luffy's Gomu Gomu no Elephant Gatling. With the threat over, Straw Hat and Whitebeard Alliance disbands, with Luffy promising to meet Ace (and Whitebeard depending on the ending) at the top and they part ways and the Straw Hats continue on their adventures.

Trivia

 * The World Government fears that if at least two Yonko ally together, they could cause an upset in balance.
 * Interestingly, Buggy urged his captain Gol D. Roger to accept an alliance with Shiki though his cries fell on deaf ears as his Captain enjoyed his freedom too much to consider an alliance. Years later as a Pirate Captain himself, Buggy would strike up an alliance with Alvida, meaning that in contrast to his former captain, Buggy seems to favor alliances between crews (especially if they have a common enemy).