"The Eighty-Six's closest companion, as well as their final resting place." ― Volume 1 Chapter 2
The M1A4 Juggernaut (ジャガーノート, Jagа̄nо̄to?) is the sole Feldreß of the Republic of San Magnolia, manufactured by Republic Military Industries. While developed as an answer to the hordes of advanced drones of the Giadian Empire's Legion, the Juggernaut is markedly inferior to the various types of Legion in firepower, protection, and mobility. Its classification as an unmanned weapon is an ostensible one, as the machines are piloted by the Eighty-Six, who are deemed non-humans under the Republic's discriminatory policies.
A Juggernaut in current English usage is a literal or metaphorical force regarded as merciless, destructive, and unstoppable. The word originates from the Hindu deity Jagannath. Its designation of M1A4 stands for Model 1 Alteration 4.
The silhouette of the Juggernaut resembled that of a prowling spider, but the two grappling arms and its brandished main battery are reminiscent of a scorpion's tail and pincers. It is a machine with four long, jointed legs with whitish-brown aluminum-alloy armor. The legs are fitted with oscillation sensors to detect approaching ground enemies and its maneuverability is slightly superior to that of a treadmill tank, the grappling arms also granted the Juggernaut vertical maneuverability, allowing them to scale buildings or cliffs to gain higher ground.
A Juggernaut's cockpit is similar to a jet fighter's, it has a clamshell cockpit and the inside is filled with a multitude of switches, LCD screens, and twin control sticks to the left and right. The biggest difference was that instead of a bulletproof glass windshield, the Juggernaut's cockpit was enclosed by an armored canopy, so the pilot could not see outside the unit. To compensate for that, the cockpit was equipped with three screens and a holo-window that provided all data of the battlefield. However, the resolution for these displays is extremely low, which leads to low crew visibility and awareness. Due to the campiness and darkness of the cockpit, Processors often nickname it as the 'Coffin'.
For the Processors piloting the Juggernaut, there was little chance of surviving a hit. Since the Juggernaut was classified as a drone, it lacked any sort of life-preserving equipment such as airbags . Moreover, its armor was so fragile that it can be penetrated easily by all the combat types of Legion. In fact, the cockpit of the Juggernaut is morbidly referred to by Processors as "the Guillotine" for its tendency to snap off from the rest of the machine when struck. The resulting injuries are often fatal because there are no cushions to protect the body of the Processor from striking against the insides of the metal cockpit.
For its main armament, the Juggernaut is equipped with a magazine-fed 57mm smoothbore gun that can fire APFSDS and HESH rounds. The APFSDS - Armour Piercing Fin-Stabilized Discarding Sabot projectiles are used against heavy armored Legions such as the Löwe. The HESH - High Explosive Squash Head rounds are used against light armored Legions such as the Ameise, and against fortifications like concrete bunkers and gates. The magazine of the gun can be quickly swapped out and reloaded by an M101 Barrett scavenger drone. For its secondary armaments, the Juggernaut is equipped with two 12.7mm heavy machine guns. On top of it, two sets of wire anchors are also fitted to a unit for various uses on the battlefield.
During the initial production, High-frequency blades were introduced as an optional weapon that could be placed on the grappling arms instead of the 12.7 mm heavy machine guns. They had enough force to even cut into the tough armor of the Dinosauria like it was made of butter.  Nonetheless, almost no processor used high-frequency blades as it required Juggernaut to sprint through gunfire and bombardment to close in on the enemy.
Unlike the more complicated hexapedal systems of the Ameise and Grauwolf and the octopedal systems of the Löwe and the Dinosauria, the Juggernaut use a quadrupedal movement system, as a result of the Republic's inept engineering capabilities being unable to develop a sufficiently powerful power pack or an adequate ECU to support a more complex drivetrain. Because of the weight constraints of the underpowered quadrupedal design, the Juggernaut could only have light armor and small calibers guns. Another drawback is that the Juggernaut has a much higher ground pressure than the Legion units due to its weight being distributed to only four legs. As a result, The processors of the Juggernaut must avoid soft, marshy terrain otherwise risk getting stuck.
In addition to its inferior movement system, the Juggernaut is also a very fragile machine, unable to undergo rapid maneuvers for very long without breaking down, limiting the extent to which Processors can use the Juggernaut's maneuverability. The cruising speed of the Juggernaut itself is 60km/h, slower than even a Löwe. The Juggernaut does make up for its inferior speed through its multi-directional mobility, more specifically vertical mobility, which it uses to exploit weak spots above most Legion units.
At the start of the war with the Giadian Empire, the technological inferiority of the Republic of San Magnolia was made clear. Within half a month, the Republic Army was decimated and its remnants forced to using delaying tactics to prolong the war. It was necessary to produce a new weapon that could fight the Legion's overwhelming numbers.
By the end of the first year of war, Republic Military Industries completed their development of a new vehicle, which was based on technology recovered from Legion wreckage, and quickly started production. However, the new M1A4 Juggernaut was a rushed design, resulting in it being inferior in many aspects to the Legion. Despite its flaws, the lack of a better machine made the Juggernaut the primary weapon of the Republic.
Unknown to the Republic citizens, who knew of the Juggernaut as an autonomous drone, the Juggernaut has to be piloted by Eighty-Six since the Republic AI technology is simply inadequate to develop a serviceable AI that can effectively counter the Legion. Since the Eighty-Six are considered non-humans, the Republic is able to fabricate the lie that the Juggernaut is not manned by humans and that there are no human casualties in the ongoing war. The use of the term Processor to refer to the Eighty-Six pilots contributes to the Republic's lies.
The Juggernaut remained the Republic's primary weapon until the Fall of the Gran Mur and in the following siege. Soon after the liberation by the Federal Republic of Giad, the surviving processors switched to the more advanced XM2 Reginleif.
Owing to the sub-par specifications of the Juggernaut, a variety of tactics for dealing with the Legion have been passed down through the ranks of Processors. When a Legion incursion is spotted, the squadrons will set ambushes on the predicted path of the Legion units. Achieving the element of surprise is crucial because it allows the Juggernauts to target the weaker rear armor of the otherwise impenetrable Löwe as well as weaken the enemy without risk of damage.
Once the first volleys are fired, the Juggernauts employ fire and maneuver tactics to temporarily suppress the Legion and allow other Juggernauts to flank the enemy positions. Using team maneuvers is especially necessary against the heavy armoured Löwe, although a few exceptional Processors such as Shinei Nouzen are capable of handling a Löwe alone. These Processors utilize high mobility tactics to take on the Löwe alone, but due to the fact that the Juggernauts are poorly designed and made, the moves that the Processors execute can seriously damage the Juggernaut itself. This poses a lot of stress on the maintenance of the Juggernaut. This is the reason why the maintenance team of the Spearhead squadron always reserves two to three operational Juggernauts for Shin. Attacking from elevation is also a preferred method against the Löwes and Grauwolfs, both of which have difficulty elevating their weapons to target enemies above them.
The presence of the Legion's Skorpion artillery units is a significant danger to Juggernauts as their light armor can be pierced by shrapnel from the high-explosive shells. To prevent an artillery strike, the Juggernauts prioritize the elimination of the Ameise, which serves as forward artillery observers for the Scorpions.
Selecting terrain is another crucial aspect of tactics as soft ground like swamps can be a deathtrap for Juggernauts. Processors scrounge for maps from the surrounding ruins to determine favorable terrain.