The Paradis Island Operation, also known as the plan to retake the Founder or the Operation to Retake the Founding Titan, was a long-term strategy undertaken by the Marleyan military and its Warriors 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. 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 Bertholdt Hoover, the Colossal Titan who would destroy the outer gate of Shiganshina District; and Reiner Braun, the Armored Titan who would destroy the inner gate, as well as Annie Leonhart, the Female Titan, and Marcel Galliard, the Jaw Titan. 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 Bertholdt Hoover, the Colossal Titan who would destroy the outer gate of Shiganshina District; and Reiner Braun, the Armored Titan who would destroy the inner gate, as well as Annie Leonhart, the Female Titan, and Marcel Galliard, the Jaw Titan. At some point before the breach, the Warriors came across a Titan who nearly caught and consumed Reiner.
Struggle for Trost: In the year 850, Bertholdt Hoover transformed once again into the Colossal Titan outside of the southern gate of Trost District, successfully breaching Wall Rose. Eren Jaeger attempted to kill the Colossal Titan, but Bertholdt managed to escape successfully, using omni-directional mobility gear to flee the scene under the cover of his Titan's steam. They brought their mission to a halt upon discovering a Titan openly fighting other Titans.
Source: Paradis Island Operation (Anime), available under CC BY-SA 3.0.