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.

Problems
A common problem with Pirate Alliances is betrayal.

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

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.

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).

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. 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. However, the true reason for the alliance was to destroy the SMILE Factory that belonged to the Shichibukai Donquixote Doflamingo who allied with Kaido. 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.

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 an unnamed Yonko. Despite some minor altercations, the three were eventually able to come to terms, and their alliance was reported in the newspaper.