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The Garrison Regiment (駐屯兵団 Chūton Heidan?) is the largest of the Military divisions which protects and maintains order within the Walls. Given the fatality rate of the Scout Regiment and the limited entry of the Military Police Regiment, most soldiers join the Garrison. As they often have close contact with both Titans and human civilians, they are proficient over a wide variety of tasks such as Titan combat (defensive battles), cannons, security detail, crowd control, and rifle usage.
The Garrison takes care of all defences for the Walls whether it be patrols, repairs, or improvements. As such, they spend most of their time preparing for attacks against the Wall. The Garrison mans the cannons on the Wall that are often used for long-range Titan defense. They must also deal with civilian evacuation and have emergency plans for dealing with scenarios for when the Walls are breached to make sure the citizens are evacuated safely.
The Garrison is the largest of the three branches, with a total of 30,000 soldiers. Members of the Cadet Corps are considered in this total until their graduation and enlistment in a chosen branch. Their total number of combat-ready personnel is a mere 3rd of this total, at 10,000 soldiers.
Their structure consists of Regiments, Brigades, and Divisions. A regiment is the smallest unit, consisting of 1000 soldiers. A Brigade consists of anywhere between 2,000 and 3,000 soldiers, or 2-3 regiments. And finally, the largest units are Divisions, which consist of 2 Brigades.
The Garrison also has an Engineering Corps who are in charge of refueling, ammunition, and other aiding jobs. The Engineering Corps does not have an insignia, they instead wear the same uniform as regular Garrison soldiers. In case of many casualties during a battle, the Engineering Corps will be deployed. During the struggle for Trost, the Engineering Corps was in the Headquarters refueling ODM gear gas tanks.
Being the main fighting force of the omni-directional mobility gear soldiers, the men and women of the Garrison come from all backgrounds and views. Some of those who joined are afraid of dying or unwilling to give their lives for what they believe to be foolish causes. Others are devoted to the protection of mankind, or joined to protect loved ones from the threat of Titans. The Garrison has also given rise to imaginative and magnetic men like Dot Pyxis, who can successfully motivate their subordinates to do their duty in the face of incredible odds.
Before the fall of Wall Maria due to a horrific Titan raid, the quality of the troops was divided, lazy, undisciplined, and unreliable; only slightly better than the Military Police Regiment. Like them, most joined just to avoid working in the farms and fields and enjoyed drinking alcohol on duty.
All of that changed after Wall Maria fell, when the Garrison faced the problems of doing their job as soldiers. Since then, the Garrison has become innovative, tactical/strategic, and disciplined. They also led the mission to seal the gate and recapture Trost, which was successful, and was humanity's first ever victory against the Titans. This branch is currently the second most reliable military branch along with the Scout Regiment.
Members of the Garrison usually work by themselves but will assist the Scout Regiment when needed, as seen during the Battle of Trost, the Coup d'état, rescuing Eren from Reiner and Bertholdt, and Rod Reiss's attack on Orvud District in Wall Sina.
It has been noted by Erwin Smith that due to their contact with Titans, the southern division of the Garrison is notably more reliable and better prepared than their counterparts stationed in the interior.
The only case of members of the Garrison coming into contact with Titans will come from a direct confrontation due to a breach in the Wall. As such, their tactics are intertwined with civilian evacuation and Titan defenses. Though in special cases the Vanguard may be extremely rarely deployed beyond the Wall usually alongside the Scout Regiment to assist in operations of special need such as when Eren had been captured, the Garrison rode alongside the Scout Regiment and the Military Police to save Eren though inflicting heavy casualties.
Special Colossal Titan Response Operation
In the case of a breach, the Garrison must mobilize its men to confront the Titans coming in. If there is no way to block the breach, then direct battle must be conducted to give enough time for all civilians in the town to escape. Upon the successful evacuation of all civilians, the gate will be closed and the district will be quarantined. It is only then that all the soldiers may escape to the safety of the Wall.
The Garrison soldiers will be deployed depending on their skill level, with the most junior soldiers facing the Titans directly at the breach and the elite guarding the innermost point of the city:
- Vanguard. These soldiers are stationed at the far end of the city, where the breach is. The Vanguard is comprised mostly of regular Garrison troops, and it is their duty to slow down the advance of the Titans if they ever manage to enter the city. Soldiers usually try to avoid direct combat with the Titans in order to increase survival rates. However, the Vanguard regiment does have a small detachment of troops whose job is to engage the Titans outside of the city, after said Titans have been maimed by a barrage of cannon fire. Most of the Garrison Vanguard was wiped out during the Battle of Trost.
- Middle guard. Located between the vanguard and the rear, the middle guard is tasked with the elimination of any Titans that may have slipped past the first lines of defense (see above). The cadets — except for Mikasa Ackermann — were allocated to the middle guard squads, as they were not skilled enough to aid the rear, and would most likely prove to be a hindrance to the soldiers in the Vanguard.
- Rear guard. They are stationed in the deepest sector of the city, guarding the civilian evacuation through the city gate to the next level of the Walls. Since these soldiers are directly responsible for defending not only the lives of the civilians but also the main access point to the deeper part of the Walls, the men and women of the Garrison Rear guard are the most skilled soldiers available in the city at the time. Mikasa was called upon to join the rear guard because of her exceptional skill.
While this is going on, all attempts to stop outside Titans from entering the district short of fighting them with omni-directional mobility gear will be used. This includes methods such as cannon bombardment and harpoon nets to stop Titans in their tracks by impaling them and preventing other Titans from coming through.
Outer Wall Breach Emergency Plan
Because the Wall is so large, the Garrison simply does not have the manpower to patrol every section. That is why districts are outcroppings of the Wall - to make Titan monitoring easier as they are naturally attracted to areas of concentrated humans. When Wall Rose is supposedly breached, this plan is used.
The main priority of the Garrison is to safely transport everyone to the next secure Wall. Failure to do so will isolate and eventually lead to the settlement being overrun, and not only the settlement, but perhaps even the entire area within the Wall. Meanwhile, the breach must be located in order to plan the next move.