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The Paradis Island Operation (パラディ島作戦 Paradi-jima Sakusen?), also known as the plan to regain the Founder (始祖奪還計画 Shiso dakkan keikaku?) was a long-term strategy undertaken by Marley to take the Founding Titan from the royal family of Paradis using the Power of the Titans.
During the 830s, the nation of Marley had become aware of the diminishing power of the seven Titans obtained during the Great Titan War. In order to meet the coming need for technological progress, the authorities of Marley turned their attention to the island of Paradis and the plentiful fossil fuels it possessed. However, the island was also home to the remnants of the former empire of Eldia, ruled by the Fritz family which possessed the Founding Titan, the greatest of all Titans.
At the closing of the Great Titan War, King Karl Fritz had made the ultimatum that if Marley made any move against the remnants of Eldia, the Founding Titan would be used to unleash the countless Colossus Titans which made up the three Walls surrounding Eldia's last territory. In order to avoid the ultimatum and a direct act of war, the military of Marley chose to begin a Warrior program in which low-class Eldian citizens within Marley would offer their children between the ages of 5 and 7 to the Marleyan military, where they would train until seven would be chosen to inherit the seven Titans in Marleyan possession.
Some time later, several Eldians were chosen to inherit six of the seven Titans of Marley, the seventh remaining in the possession of the Tybur family. These six warriors were Zeke Yeager, Bertolt Hoover, Reiner Braun, Annie Leonhart, Marcel Galliard, and Pieck. Out of the six, Zeke was chosen to become the Warchief of the warriors. Annie, in particular, faced grueling combat training in her early childhood in preparation for the program, supervised by her father. However, before she departed on her mission, her father gave her his heartfelt apologies, assuring her that even if the world came to hate her, she would still have her father by her side. All that he asked of her is that she would promise to return home.
The Fall of Wall Maria
- Main article: Fall of Wall Maria
In the year 845, Marley sent four of their Warriors to the Walls with the mission of wiping out its inhabitants, taking the Founding Titan from the royal family, and putting an end to the cursed history of the people of the Walls. The four sent were Bertolt Hoover, the Colossus Titan who would destroy the outer gate of Shiganshina District; Reiner Braun, the Armored Titan who would destroy the inner gate; Annie Leonhart, the Female Titan who called forth the Titans to invade Wall Maria and the Warriors' transportation; and Marcel Galliard, the Jaw Titan who would also be transporting them to the wall.
At some point before the breach, the warriors came across a Titan who nearly caught and consumed Reiner. However, before he was caught, Marcel shoved him aside and was caught by the Titan in his place. Unable to save their friend, the warriors fled the scene as Marcel's power passed into the Titan. After the warriors had left, the Titan's human form, a girl named Ymir, awoke for the first time in around sixty years with the power of the Jaw Titan. As Marcel is lost, Annie and Bertolt decided to cancel the operation, but Reiner prevented them from stopping the operation and claimed that it would result in their deaths, and took on the role of the leader after a scuffle. As the Jaw Titan was lost, the Female Titan had to do all the transportation, pushing Annie to her limit until just before they reached the wall, at which point she switched transportation duties with the Armored Titan.
Upon arriving at Shiganshina, the southernmost district of the Walls, while Annie fell unconscious, Bertolt Hoover transformed into the Colossus Titan and broke through the south gate in a single kick. The Titans soon began to enter into the city in large numbers, feasting on the citizens and soldiers within. The Armored Titan carried everyone across the top of the wall and sneaked to the inner gate as the Eldians scrambled to evacuate the civilians. He then approached the northern gate, charging through and fully breaching Wall Maria. Reiner and Bertolt carried Annie and then infiltrated the refugees as humans, blending in with the native inhabitants of the Walls. Annie later woke up in a refugee camp inside Wall Rose.
Two years after their successful attack on Wall Maria, in the year 847, the warriors enter the 104th Training Corps in the hopes to achieve the highest ranks possible and entering into the Military Police Brigade, giving them easy access to the interior, and by extent the royal family, the holders of the Founding Titan.
During the training period, the three often discuss over their progress and their plan, with Annie continually survey for information. At the graduation, all of them succeeded in earning ranks within the top ten of their Training Corps, probably due to their Warrior training they have had before.
The Battle of Trost District
- Main article: Battle of Trost District
After receiving their desired ranks in the 104th Training Corps, the warriors proceeded with their mission on the day before their scheduled graduation. In the year 850, Bertolt Hoover transformed once again into the Colossus Titan outside of the southern gate of Trost District, successfully breaching Wall Rose. Eren Yeager attempted to destroy the Colossus Titan, but Bertolt managed to escape successfully, using vertical maneuvering equipment to flee the scene under the cover of his Titan's steam.
Before having Reiner transform into the Armored Titan to breach the north gate, the Warriors decided to wait until more Titans gathered. However, they brought their mission to a halt upon discovering a Titan openly fighting other Titans. Suspecting this Titan to be a human with the power of the Titans, Bertolt, Reiner, and Annie followed their fellow trainees' plan to use the Titan against the other Titans while they gather the resources necessary to leave the district. When the Titan was found again afterwards and collapses from exhaustion, the trainees discovered it to be Eren Yeager, having miraculously survived being swallowed by a Titan due to his newfound powers. With this unexpected turn of events, the Warriors began to suspect that Eren might have somehow inherited the Founding Titan, a possibility which would greatly simplify their mission. They halted their plans, and the inner gate of Trost District was never attacked by Reiner throughout the remainder of the battle.
Capturing Eren Yeager
When the time came for their graduation, Bertolt and Reiner changed their plans of joining the Military Police and joined the Survey Corps instead, Eren's chosen military branch. Annie Leonhart proceeded with the original plan and joined the Military Police Brigade in the Interior. However, when the Survey Corps embarked on the 57th expedition beyond the Walls, Annie invaded the expedition as the Female Titan in order to locate and capture Eren.
With Reiner's help, she nearly succeeded and managed to wipe out the Special Operations Squad, but she was foiled by Mikasa Ackerman and Captain Levi. Because she spared Armin Arlert in the attack on the expedition, the Survey Corps was able to figure out she was the Female Titan. A few days after the expedition, Annie battled with Eren in Stohess District and was ultimately caught by the Survey Corps. However, she used her specialized hardening abilities to conceal herself in an ultrahard layer of crystallized skin, preventing interrogation. Annie's comatose body was kept in the custody of the Survey Corps, signalling the loss of the Female Titan.
On the same day as the battle within Stohess District, a new Warrior arrived within the Walls. Warchief Zeke Yeager, an experienced and powerful Warrior of Marley, invaded Wall Rose. He turned the people of Ragako village into mindless Titans, perhaps in order to test humanity's strength, before leaving Wall Rose later that night. During the ensuing efforts of the Survey Corps and Garrison to deal with the event, the Survey Corps received family records from the chaos of five years ago and began to suspect that Reiner and Bertolt were in league with Annie.
A day after the beginning of the Ragako event, Reiner revealed himself and Bertolt to be the Armored and Colossus Titans respectively. Throwing all secrecy away, Reiner and Bertolt battled with the Survey Corps and captured Eren and Ymir, who they learned earlier was the Titan who ate Marcel. Over five hours later, a combined effort of all three military forces embarked on a rescue operation, and Eren was retrieved. In this battle, Eren seemingly unleashed the power of the Founding Titan upon Reiner and Bertolt, confirming their worst fears. The warriors fled with Ymir to Shiganshina District to recover  and sent Ymir back to Marley, where her Jaw Titan's power was passed on to Porco Galliard, allowing Marley to regain one of its Titans.
The Battle of Shiganshina District
- Main article: Battle of Shiganshina District
Two months after their arrival at Shiganshina, Zeke reunited with his Warriors. They expressed their desire to save Annie from the Survey Corps, but Zeke prioritized the retrieval of the Coordinate. They lay in wait in Shiganshina for the Survey Corps to arrive in search of Eren's basement, which was believed to hold secrets of the Titans. When the Survey Corps arrived, a battle ensued between the soldiers and Zeke's Warriors. The Corps, numbering over two hundred at the start of the battle, lost nearly all of its soldiers in the attack, though they succeeded in defeating Reiner and Bertolt.
In the waning moments of the conflict, Pieck rescued Zeke from Levi, but left Bertolt behind since Zeke was not prepared to face Levi so quickly again. Shortly after they abandoned Bertolt, she managed to rescue Reiner from Hange Zoë and Jean Kirstein. Shortly thereafter, Bertolt was eaten alive by a revived Armin Arlert after being administered the Titan injection, signaling Marley's loss of the Colossus Titan.
After their loss at the battle of Shiganshina District, the Warriors returned to Marley in defeat, having lost the Colossus and Female Titans to the Eldians of Paradis. These losses signaled a significant decline in the military influence of Marley, a weakness that did not go unnoticed by its neighboring countries. After receiving the news of Marley's losses, the Mid-East Allied Forces took the opportunity and declared war on Marley.