Operation to retake Wall Maria (Anime)

The operation to retake Wall Maria (ウォール・マリア奪還作戦) was a mission assigned by the central government in which twenty percent of the human population that remained after the fall of Wall Maria were sent out to reclaim 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 the third of its territory. As a consequence, many refugees came to Wall Rose and soon food was lacking.