Operation to reclaim Wall Maria

The operation to reclaim Wall Maria (ウォール・マリア奪還作戦) was a mission assigned by the Royal Government in which twenty percent of the human population that remained after the fall of Wall Maria were sent out to reclaim Wall Maria. 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.

Anime and Manga Differences
In the anime, it is said that approximately 250,000 individuals participated in the operation to reclaim Wall Maria, with less than 200 individuals surviving the mission. In the manga, it is only said that 20% of humanity's population participated in the operation, with a majority dying in the event, and no specific numbers are given.