World War III Arc

The World War III Arc is a major and the final arc of the Toaru Majutsu no Index light novels, spanning Volumes 20, 21 and 22. It is a culmination of all the previous arcs, a magic-science global war between Academy City and Russia, controlled by the Roman Catholic Church, itself under the influence of God's Right Seat. However, the war was actually part of the plan of Fiamma of the Right, manipulating the great Churches and even God's Right Seat to do his bidding.

Due to the forces that are composed in the war, it marks the first conflict that both the Magic and Science Sides have been trying to prevent since even before the start of the story.

The story arc focuses on the three protagonists, Kamijou Touma, Accelerator, and Hamazura Shiage, heading deep into Russia in order to save the one they hold dear just as the entire world plunges into a Third World War.

In order to save Index, Touma is hunting down the leader of God's Right Seat Fiamma of the Right, the man who stole the spiritual device that controlling Index as the latter required the knowledge of the 103,000 grimoires to move his grand scheme along, and now has sparked a global conflict in order to fulfill his ambitions.

Accelerator is also forced to travel to Russia to find a way to break the connection between Last Order and Aiwass, which has been causing the former and the one person he cares about immense pain by being forced to anchor Aiwass' existence to the real world through the Misaka Network.

Finally, Shiage is on the run with Takitsubo after escaping their Academy City pursuers to Russia. Shiage now hopes to find someone in Russia who could help cure Takitsubo of the Body Crystal poison, as well as find something valuable enough to negotiate their return Academy City.

Chronology
During Touma's journey however, he gains a traveling companion in the form of British New Light member, Lessar, that he rescued from death during the British Halloween. The two eventually find the God's Right Seat member's hideout along the Elizalina Alliance border, but the two are forced to split when Fiamma traps the boy aboard his flying fortress, the Star of Bethlehem. From there on out aboard the flying castle, the battle between Touma and Fiamma takes place.

Following the advice of Aiwass, which told him to find the answer he's searching for in Russia, Accelerator heads there to break the painful connection between Last Order and Aiwass. Along the way however, Accelerator comes across a batch of magic parchments that are highly valuable to his enemies, as well as the next experiment performed by Academy City's dark side, Misaka Worst, that has been sent to put an end to him. Later on, Accelerator is forced into a battle against the Fiamma controlled archangel, Misha Kreutzev, that appeared before him, and is later able to discover that the key to Last Order's cure is related to the song sung by Index during the 0930 Incident that was previously used to cure Last Order of Kihara Amata's virus.

On Shiage's side, he, and Takitsubo, are currently on the run from Academy City's pursuers after parachuting into Russia. Stealing a vehicle, the two travel by car to the Elizalina Alliance of Independent Nations in the hopes that they can find someone who can cure Takitsubo of the damage she's suffered from Body Crystal poison as well as find something to negotiate with the Academy City higher-ups in order to get back into the city. However, they are forced to make a pit stop at a Russian village run by a man named Digurv, who was currently engaged with a Russian privateer unit that had come to raid their village during the war. After helping them, Shiage finally arrives at the Elizalina Alliance, and seeks help from the actual Elizalina to get the poison out of Takitsubo with her magic powers, but knew that Takitsubo couldn't truly be healed without the top-notch technology within Academy City. Later on, Shiage is forced to deal with the threat of the Russian Kremlin Report trying to kill off everyone in the area with a bacterial weapon, and finally reached a conclusion with his long battle against Mugino Shizuri. He also finds the answer to negotiate his way back into Academy City being the Parameter List data that reveals the level limit of every esper student in Academy City.

Trivia

 * With nine episodes used for the adaptation, this would make the World War III arc the longest story arc in the anime adaptation of the Toaru Majutsu no Index Light Novels to date.