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
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.

Saruyama Alliance


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
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
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. 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 pending revocation. 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 the Will of the D.

Kid, On Air, and Hawkins Alliance
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


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.

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 to 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).