The world of One Piece is packed with many strong characters who pose a threat to the integrity of the World Government and the pirates. In certain cases, it is impossible to defeat these strong characters without some external help.
To advance the story, it is important that these characters be defeated. In order to take down these powerhouses, several characters band together and form an alliance. There have already been many strong alliances in One Piece, but some of them are a cut above the rest, and they deserve all the praise that they get.
7 Straw Hats and Fire Tank Alliance
After learning about Sanji's decision to leave the crew, Luffy and a few other members of the Straw Hat Pirates decided to head to Whole Cake Island to retrieve him. Upon their arrival, the Sanji Retrieval team encountered multiple threats, and eventually, they met the Fire Tank Pirates.
Like the Straw Hat Pirates, the Fire Tank Pirates are led by a Supernova, namely, Capone Gang Bege. Luffy and co. had a meeting with Bege, who told them about his plan to assassinate Big Mom. On paper, the plan was perfect, but they underestimated the power of Big Mom. The alliance was fairly powerful, but the Straw Hats were missing some of their biggest heavy hitters, so it is ranked the lowest.
6 Impel Down Escapees Alliance
When Luffy was sent to Amazon Lily by Bartholomew Kuma, he had to fight for his life against Boa Sandersonia and Marigold. Shortly after the fight, Luffy learned about Ace's execution. Without wasting any time, he asked Hancock to help him invade Impel Down. Overcome by love, Boa Hancock agreed and sneaked Luffy inside Impel Down.
Inside the prison, Luffy had to overcome many hurdles before he finally reached the lowest level. When he arrived at his destination, he was met with disappointment, as Ace had already been transferred to Marineford. Luffy couldn't get out on his own, but luckily he met Ivankov, Bon Clay, and several other prisoners who were willing to help him break out of Impel Down. The alliance remained in effect until they reached Marineford, after which most of them went their separate ways.
5 Straw Hats and Heart Pirates Alliance
The Straw Hats left Fish-Man Island on a high, and they finally entered the New World. The crew's first stop was Punk Hazard, an island formerly under the control of the World Government. On the island, the Straw Hats met Trafalgar Law, who had managed to become a Shichibukai.
It was odd to see Law on a restricted island, but soon he revealed his intention to take down Kaido. Luffy accepted Law as his ally, and this led to a strong alliance that lasted until the end of the Wano Country arc. Afterward, it was made clear that Law's main goal was to take down Donquixote Doflamingo.
4 Shiki's Pirate Armada
Golden Lion Shiki is one of the legendary figures in the One Piece world. He used to be a member of the Rocks Pirates before they separated. Following their disbandment, Shiki started the Golden Lion Pirates and made a name for himself.
Many pirate crews joined Shiki and supported his goal of world dominance. Once Shiki gained control over a massive fleet of ships, he became extremely feared. Shiki and his armada fought the Roger Pirates in the Edd War. Needless to say, the Roger Pirates would have lost if it weren't for the torrid weather.
3 Marines-Roger Pirates Alliance
Dabbling in the history of One Piece is a serious offense, and anyone who goes against the rules is punished severely by the World Government. Many decades prior to the start of the story, a pirate named Rocks D. Xebec gathered a formidable crew that became the bane of the World Government.
Rocks wanted to become the "King of the World," so he tried to uncover the secrets of the World Government. Considering his immense power, the Marines led by Monkey D. Garp decided to form an alliance with the Roger Pirates to take down the Rocks Pirates. The battle took place in God Valley, and the alliance emerged victorious. The Rocks Pirates disbanded, and their captain was reported dead.
2 Kaido-Big Mom's Alliance
Kaido and Big Mom were two of the Yonko, which meant that they were among the strongest characters in the world. Big Mom's crew consisted of talented fighters such as Charlotte Katakuri, Oven, Daifuku, Smoothie, and Cracker.
Kaido was considered to be the "Strongest Creature in the World," and he was the captain of the infamous Beasts Pirates. His crew had many powerful individuals, which included the likes of King, Queen, Jack, Who's Who, and Basil Hawkins. The news about the two Yonko forming an alliance shocked the entire world, and it caused the Marines to remain on high alert.
1 Pirate-Ninja-Mink-Samurai Alliance
The Pirate-Ninja-Mink-Samurai alliance was formed with only one goal in mind: to defeat Kaido and end his reign of terror. Originally, it was the Strawhat-Heart Pirates alliance, but as they recruited more and more members, it expanded, and it was named the Pirate-Ninja-Mink-Samurai alliance.
The alliance had many notable members, which included Luffy, Law, Kid, Denjiro, Yamato, Marco, etc. Despite their inferior numbers, the alliance managed to overwhelm both Kaido and Big Mom. This was a monumental victory for all the characters involved in the war. As the news of the defeat of the two Yonko spread, the entire world was shocked. All the pirates who were a part of the alliance had their bounty increased.