Paradis Island Operation

The plan to regain the Founder (始祖奪還計画) was a long-term operation undertaken by Marley to take the Founding Titan from the royal family of Paradis using the power of the Titans.

Prelude
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 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: Zeke Yeager, Bertolt Hoover, Reiner Braun, Annie Leonhart, Marcel, and Pieck. Out of the six, Zeke was chosen to become the warchief of the warriors, likely due to his age and royal Fritz blood inherited from his mother Dina Fritz. 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 Plan
In the year 845, Marley sent Titan shifters to the Walls with the mission of wiping out its inhabitants, taking the Coordinate from the Reiss family, and putting an end to the cursed history of the people of the Walls. The four known Titan shifters 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 presumably called forth the Titans who would invade Wall Maria; and Marcel whose Titan form and purpose are currently unknown.

When they arrived at Shiganshina District, the warriors succeeded in breaching both gates and sending the Titans swarming into humanity's territory. However, before Marcel could succeed in whatever task he was given, a mindless Titan attacks Reiner. He sacrifices himself to save the others, being consumed by the Titan. As the other warriors flee, this Titan inherits Marcel's Titan shifting power and returns to her human form, Ymir.

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 Reiss family, the holders of the Coordinate. Reiner, Bertolt, and Annie succeeded in earning ranks within the top ten of their Training Corps.

After receiving their desired ranks in the 104th Training Corps, the warriors proceed with their mission on the day before their scheduled graduation. In the year 850, Bertolt Hoover successfully breaches the outer gate of Trost District. However, the warriors bring their mission to a halt when they discover a Titan who is openly fighting other Titans, later revealed to be Eren Yeager with newfound Titan shifting abilities. With this unexpected turn of events, the warriors begin to suspect that Eren might have somehow inherited the Coordinate. They halt their plans, and the inner gate is never attacked by Reiner.

When the time comes for their graduation, Bertolt and Reiner change their plans of joining the Military Police and instead join the Survey Corps, Eren's chosen military branch. Annie Leonhart proceeds with her original decision to join the Military Police Brigade in the Interior. However, when the Survey Corps embarks on the 57th expedition beyond the Walls, Annie invades the expedition as the Female Titan in order to locate and capture Eren for the Hometown. With the help of Reiner she nearly succeeds and manages to wipe out the Special Operations Squad, but is foiled by Mikasa Ackerman and Captain Levi. Because she spared Armin Arlert in the attack on the expedition, she is found out to be the Female Titan. A few days after the expedition, Annie battles with Eren in Stohess District and is ultimately caught by the Survey Corps. However, she uses her specialized hardening abilities to conceal herself in an ultrahard layer of crystallized skin, preventing interrogation. Annie's comatose body is kept in the custody of the Survey Corps.

On the same day as the battle within Stohess District, a new warrior arrives within the Walls. Warchief Zeke Yeager, an experienced and powerful warrior of the Hometown, invades Wall Rose, turning 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 receives family records from the chaos of five years ago and begin to suspect that Reiner and Bertolt were in league with Annie. A day after the beginning of the Ragako event, Reiner and Bertolt are indeed discovered to be the Armored and Colossus Titans. Throwing all secrecy away, Reiner and Bertolt battle with the Survey Corps and capture Eren and Ymir, who they learned earlier was the Titan that ate Marcel. Over five hours later, a combined effort of all three military forces embarks on a rescue operation, and Eren is retrieved. In this battle, Eren seemingly unleashes the Coordinate upon Reiner and Bertolt, confirming their worst fears. The warriors flee with Ymir to Shiganshina District to recover and formulate a new plan. Ymir eventually leaves the group, and her fate is unknown.

Two months after their arrival at Shiganshina, Zeke reunites with his warriors. They express their desire to save Annie from the Survey Corps, but Zeke prioritizes the retrieval of the Coordinate. They lie in waiting in Shiganshina District for the Survey Corps to arrive in search of Eren's basement, which is believed to hold secrets of the Titans. When the Survey Corps arrives, a battle ensues between the soldiers and Zeke's warriors and Titans. The Corps, numbering in over one hundred at the start of the battle, loses nearly all of its soldiers in the attack, though they succeed in defeating Reiner and Bertolt. However, in the last moments of the conflict, Zeke uses Pieck as a mount to escape from the battle with Reiner, leaving Bertolt to the mercy of the Survey Corps. Shortly thereafter, Bertolt is eaten alive by a revived Armin Arlert after being administered the Titan serum, signaling the loss of the Colossus Titan.

Aftermath
After their loss at the battle of Shiganshina District, the warriors returned to Marley in defeat, having lost two of their Titans to the Eldians of Paradis. These losses signaled a significant decline in the military influence of Marley, a weakness that would be exploited by the Mid-East Allied Forces. After news of Marley's losses became known to the Allies, war was declared on Marley.