The operation to reclaim Wall Maria was a mission assigned by the Royal Government in which twenty percent of the population that remained after the fall of Wall Maria were sent out to reclaim the lost land. In truth, the operation was begun in order to alleviate the food shortage brought on by the significant loss of territory and influx of refugees from Wall Maria, and a tremendous number of people died during the operation at the hands of the Titans.
Prelude: After the fall of Wall Maria, humankind lost more than one third of its territory. As a consequence, many refugees came to Wall Rose and soon food was lacking.
The operation: Under the pretext of a plan to recapture Wall Maria, twenty percent of the population within the Walls, many of them refugees, were enlisted by the Royal Government and sent outside the Walls.
Aftermath: The majority of the people sent outside the Walls died. However, the sacrifice of a large portion of the population helped ease the food shortage for the rest of the people within the Walls.
Source: Operation to reclaim Wall Maria, available under CC BY-SA 3.0.