The Paradis Island Operation, also known as the plan to retake the Founder, was a long-term strategy undertaken by the Marleyan military and its Warrior Unit to take the Founding Titan and eradicate the people of the Walls.
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 Paradis Island 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.
The operation: 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 would call forth the Titans to invade Wall Maria and the Warriors' transportation; and Marcel Galliard, the Jaw Titan who would also be taking them to the wall. At some point before the breach, the Warriors came across a Titan who nearly caught and consumed Reiner.
The 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 would call forth the Titans to invade Wall Maria and the Warriors' transportation; and Marcel Galliard, the Jaw Titan who would also be taking them to the wall. At some point before the breach, the Warriors came across a Titan who nearly caught and consumed Reiner.
The 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 kill 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.
Source: Paradis Island Operation, available under CC BY-SA 3.0.