The Battle of Marinford was a major armed conflict between several world powers of the One Piece universe. It occurred 22 years after the death of the Pirate King Gol D. Roger on the isle of Marinford in Grand Line. The Marines had incurred the ire of the Edward Newgate (Whitebeard) by attempting to execute the 3rd Division Commander of the Whitebeard Pirates, Portgas D. Ace, and found the execution interrupted by a pirate invasion. The result was a total defeat for the Whitebeard Pirates, with both Whitebeard himself and Ace perishing in the conflict, and major losses to the armies of both the Marines and the Whitebeard Pirates. The Battle of Marinford is the first time Eiichiro Oda has ever shown the death of a major character in real-time, with the majority of deaths occurring before the present timeline, recounted as flashbacks.
Background
Several weeks before the conflict, Marshal D. Teach (later known as Blackbeard), sought access to the hidden dungeons of the great prison Impel Down. He had hoped to recruit powerful allies from the great criminals entombed there to support his quest to become the next Pirate King. Rather than assault the seemingly impenetrable prison directly, Teach instead pursued Shichibukai candidacy, which had a vacancy following the ejection of Sir Crocodile due to Crocodile's involvement with the Alabasta Civil War. As a Shichibukai, Teach would be able to enter the prison's dungeons without suspicion, providing him the opportunity to free his would-be allies. Because Teach was an unknown pirate with no bounty or the name recognition desired of a Shichibukai, he was searching for a pirate in the Old World who had a bounty of over 100 Million Belly, who he would turn over to the World Government as proof of his abilities.
Teach encountered Monkey D. Luffy on Jaya Island, and after learning he had a 100 Million Belly bounty pursued his head. While resting on Banaro Island, Teach was finally tracked by his former commander in the Whitebeard Pirates, Portgas D. Ace. In the ensuing Battle of Banaro Island, Ace was defeated and captured by Teach, then handed over to the Marines. With support of the World Government, the Marines announced a public execution for Ace. As word of the conflict spread around the world, the Whitebeard Pirates mobilized a strike force to rescue Ace. In response to this threat, the World Government issued a mandatory subpoena for all active Shichibukai to support the Marines.
With aid from Shichibukai Boa Hancock, Monkey D. Luffy successfully invaded Impel Down but failed to rescue Ace before Ace was escorted to the execution site at Marinford. In the process, he recruited many Impel Down inmates, including Buggy the Clown, several former officers of Baroque Works, and a small division of the Revolutionary Army to support his objective. Blackbeard coincidentally arrived toward the end of Strawhat Luffy's assault, and after a brief skirmish with Strawhat and Warden Magellan, successfully freed many of the powerful/dangerous criminals from Impel Down's notorious floor six.
Parties
On the side of the Marines were all of the great present day naval figures. Fleet Admiral Sengoku,
Phase I
The Marines (Commander: Fleet Admiral Sengoku) Shichibukai (Mihawk, Donquixote de Flamingo, Gecko Moria, Bartholomew Kuma) Pacifista (Commander: Sentomaru) Blackbeard Pirates (Commander: Marshal D. Teach)
Whitebeard Pirates (Commander: Edward Newgate) Straw Hat Pirates (Commander: Monkey D. Luffy) Revolutionary Army (Commander: Emporio Ivankov) Buggy Pirates (Commander: Buggy the Clown) Baroque Works (Commander: Sir Crocodile) Shichibukai (Jimbei, Boa Hancock)
Phase II
The Marines (Commander: Fleet Admiral Sengoku) Shichibukai (Mihawk, Donquixote de Flamingo, Gecko Moria, Bartholomew Kuma) Pacifista (Commander: Sentomaru)
Whitebeard Pirates (Commander: Marco) Blackbeard Pirates (Commander: Marshal D. Teach) Straw Hat Pirates (Commander: Monkey D. Luffy) Revolutionary Army (Commander: Emporio Ivankov) Buggy Pirates (Commander: Buggy the Clown) Baroque Works (Commander: Sir Crocodile) Shichibukai (Jimbei, Boa Hancock) Heart Pirates (Commander: Trafalgar Law) Red Hair Pirates (Commander: Shanks)