57th Expedition Outside the Walls

The Dai 57 Kai Hekigai Chōsa (第57回壁外調査) was a mission undertaken by the Survey Corps to plot a course from Karanes District to Shiganshina District. The expedition was sabotaged by the Female Titan.

Military Tribunal
In the days following the battle of Trost District, Eren Yeager's Titan powers had become public knowledge. The interior districts in particular fell into a state of general confusion over the news, highly skeptical of the claims presented by both the Royal Government and the civilian merchants. Those in positions of authority within the government regarded Eren as a threat, though they publicly abided by their policy of non-intervention in matters not concerning Wall Sheena. On the other hand, many saw Eren as a hero for his actions, and tensions rose between the people of Wall Sheena and Wall Rose. At this time, the threat of civil war was being considered by Military Police Brigade Commander Nile Dok.

At this time, Eren's Titan abilities also became known to Rod Reiss, the last known living member of the Reiss family, the true royal family living in secret as nobles in Wall Sheena. Understanding that Eren Yeager must have been the holder of the Coordinate, the power of the Reiss family stolen five years ago, orders were given to the Military Police that Eren must be taken alive into their custody.

Shortly after Eren awoke from his coma, a military tribunal led by Commander-in-Chief Darius Zackly was held in order to determine the fate of Eren Yeager. Several merchants, representatives of the Church of the Walls, and other political figures were present at this time. Commander Nile Dok of the Military Police spoke his concerns of civil war, claiming that Eren would be examined thoroughly before being disposed of. Commander Erwin Smith of the Survey Corps proposed inducting Eren into the Survey Corps as a full-fledged member with his Titan powers being put to use in reclaiming Wall Maria. Tensions arose in the courtroom, though eventually it was decided that Eren would be handed over to the custody of the Survey Corps under the condition that his powers would be used in the next expedition beyond Wall Rose. If the expedition were to fail, Eren's fate would once again be at the military's discretion.

Preparations for the Expedition
After officially joining the Survey Corps, Eren was placed under the command of Captain Levi Ackerman, who had handpicked a selection of elite Survey Corps soldiers known as the Special Operations Squad to guard Eren and kill him if necessary. After cleaning and refurbishing an abandoned Survey Corps fortress, Eren was introduced to Squad Leader Hange Zoë, who would be in charge of performing experimentation on Eren's Titan abilities. Hange informed Eren of the Survey Corps' findings in the investigation of "Sonny" and "Bean", the two Titans that had been captured in the final moments of the battle of Trost.

At this time, Annie Leonhart infiltrated the Survey Corps grounds and killed Sonny and Bean in order to hinder the Survey Corps' research. Members of the 104th Training Corps were soon investigated concerning the matter, and Annie presented Marco Bott's vertical maneuvering equipment in order to pass the examination, though this was noticed by her fellow trainee Armin Arlert. Later that day, Erwin Smith held a speech offering to accept new members into the Survey Corps, making clear their intentions of reaching Shiganshina District and locating Grisha Yeager's basement which held the secrets of the Titans. Twenty-one trainees joined the Survey Corps that night, including Marley warriors Reiner Braun and Bertolt Hoover. Their comrade Annie instead left to join the Military Police Brigade. The next day, the new recruits met Eren at the castle and informed him of Marco's death.

In the month following the Survey Corps acceptance ceremony, Eren remained under the watch of the Special Operations Squad as Hange performed various tests on Eren's Titan form. Tensions arose when Eren seemingly activated his Titan powers without permission, though it was later discovered that this event was in fact an effect of his Titan abilities. Hange concluded that not only was self harm necessary for Titan transformation, but also a fixed goal in the mind of the Titan shifter.

During these weeks, other new members of the Survey Corps were trained in the strategy that would be implemented in the upcoming expedition: the Long-Distance Enemy Scouting Formation. In this formation, the Survey Corps soldiers on horseback would be spread across a great distance in a semicircular form divided into multiple columns. New recruits would be trusted with communications and spare horse maintenance. With this strategy, scouts at the front of the formation would warn the formation of approaching Titans with red or black signal flares, depending on whether it is a common Titan or an Abnormal, respectively. Upon seeing the flare, the command group at the front of the formation would fire a green flare in the new direction in order to avoid contact with the Titan sighted.

Before the start of the expedition, Commander Erwin Smith revealed one of the hidden motives behind the expedition to the less-than-twenty surviving members of the Survey Corps from before the fall of Wall Maria. While the official purpose of the expedition would be to plot a new course from Karanes District in the east to Shiganshina District, Erwin had begun to suspect a Titan spy with powers similar to Eren was hiding within the military. Using the newly developed special target restraining weapon, the veterans planned to make use of the device to restrain and capture an intelligent enemy Titan should one infiltrate the expedition. For secrecy's sake, this plan was not made known to the Survey Corps soldiers outside of the survivors from five years prior, who could not have been spies.

With the Trost District gate sealed permanently, the Survey Corps made its way to Karanes, the eastern district of Wall Rose in order to plot a new path to Shiganshina.