The Summit War Saga, often referred to as the Paramount War Saga, is one of the largest storylines in Eiichiro Oda's manga and anime series One Piece. It comprises five different arcs: Sabaody Archipelago, Amazon Lily, Impel Down, Marineford, and Post-War. These combine together to form one of the most significant and developmental periods in the series, hugely changing the world of One Piece and the path it will take its characters on.
The saga opens at the Sabaody Archipelago Arc, where the Straw Hat Pirates get separated by Bartholomew Kuma, one of the Seven Warlords of the Sea. In this arc, a number of important characters are introduced that include Silvers Rayleigh, who used to be Gol D. Roger's first mate, the Pirate King. Rayleigh's appearance gives very important information about the world of One Piece, even though Luffy is rather famous for his declining attitude when learning about the titular treasure to discover it himself.
Following the crew's separation, the Amazon Lily arc deals with Monkey D. Luffy's arrival in the country of women governed by Boa Hancock. This arc raises considerations pertaining to issues of gender and highlights Hancock as one of the complex characters who turned out to be very important to Luffy.
The Impel Down arc is the one in which Luffy sneaks into the world's most secure underwater prison to rescue his brother, Portgas D. Ace. This arc conspicuously marks the revival of a good deal of former foes as unlikely allies, with Buggy the Clown and Mr. 2 Bon Clay being part of it. Above all, the fights inside Impel Down raise the stakes and dangers of Luffy's journey, especially Luffy against the poison-user Warden Magellan.
The center of the saga is the Marineford arc, which portrays an epic war between the World Government and the Whitebeard Pirates. This war is fought to stop Ace's execution and contains the largest collection of powerful people ever in the One Piece World: the three Admirals of the Marines, Akainu, Aokiji, and Kizaru; all the Seven Warlords; and the Whitebeard Pirates with their leader, Edward Newgate, one of the Four Emperors. What makes the Marineford Arc so famous is the enormous battles and huge character deaths, like Luffy's father, Dragon, being a leader of the Revolutionary Army.
The final arc, which closes this saga of the Summit War, is the Post-War Arc, where it is described what happens after the battle. Still in shock from the events on Marineford, Luffy receives a message from Rayleigh during the arc to train for two years before he reunites with his crew. This marks the huge milestone of the series since it will turn into a timeskip and meaningful character development for all of the Straw Hat Pirates.
Many themes are key throughout the saga. The question of justice is focused on through the actions of the Marines and the World Government, contrasting their "Absolute Justice" with the pirates' quest for freedom. The saga further develops the world's past, mostly in relation to Gol D. Roger and his Will of D.
While every saga of One Piece has its unique glory, the Summit War Saga stands out. Most of the sagas are focused on the entire Straw Hat crew, but this one focuses so much on Luffy alone. The development of Luffy's character, new allies, and enemies has further intense development in this format. The reception from the fans was pretty mixed since many felt this format refreshing, while others missed the dynamic of the crew like in the previous arcs.
In terms of world-building, the Summit War Saga highly expands the scope of the One Piece world. It fully details the concept of Haki with its different forms and its importance in high-level fighting. This saga also gave a better explanation on the power scale of the world, just showing the power of the Four Emperors and the extent of influence the World Government has.