The Corps Collapses

Kairan no Heidan (潰乱の兵団) is the 4th chapter of the 5th volume and the 17th chapter overall of the Attack on Titan: Before the Fall manga, written by Ryō Suzukaze and illustrated by Satoshi Shiki.

Summary
Jorge Pikale tells Kuklo, Cardina Baumeister, and Sharle Inocencio about an expedition to capture a Titan that he led fifteen years prior. He also reveals that two of the soldiers on the expedition, Sorum Humé and Carlo Pikale, were present for Kuklo's birth, and that the inventor of The Device, Angel Aaltonen, accompanied the Survey Corps during the expedition.

Fifteen years ago, the Survey Corps has suceeded in capturing a Titan. Staring at the Titan that the Corps has restrained, Angel comments that the more he watches it, the more human it looks. As Angel approaches the Titan for a closer look, it tries to lash out at him, but is held back by the net holding it. Jorge observes that Angel's Iron Bamboo net is proving successful in holding down the Titan, and Angel, who has been inspecting the Titan, comments that its skin is warmer than a human's. On Jorge's orders, Carlo and Sorum begin attacking the Titan with Iron Bamboo daggers, searching for a weakness. As Carlo and Sorum continuously cut the Titan, a disturbed Angel notes that the Titan is not responding to the pain.

Carlo and Sorum succeed in cutting off one of the Titan's legs, and Angel and a Corps soldier observe, after inspecting it, that the Titan's bone structure and muscles are very similar to that of a human. Jorge orders the soldiers to the Titan's leg for further investigation, but the leg dissolves in a burst of steam before they can retrieve it. As everyone stares in shock, Sorum directs their attention to the Titan, whose leg is in the middle of regenerating.

Although the soldiers are disheartened, Jorge encourages them, telling them that the Titan's weakness must simply not lie below its knees. He decides, however, that severing limbs with blades will take too long, and has the soldiers fasten explosives to the Titan. The soldiers use their explosives to blow off the Titan's right arm and leg, but they quickly regenerate. As Angel, Carlo, and Sorum attach more explosives, Angel notes that the Titan does not possess reproductive organs.

When the Titan regenerates its limbs again, Angel grows agitated and attacks the Titan in a fit of rage, stabbing it in the face multiple times, before dropping an explosive down its mouth. An astonished Sorum manages to pull Angel away before the explosive detonates, and as he watches the Titan's damaged form, Angel angrily tells it that what they're doing is nothing compared to what the Titans have done.

In the present, Jorge explains to Kuklo, Cardina, and Sharle that Angel's anger was due to his assistant, Corina, being devoured by the Titan that was let into Shiganshina District.

During the expedition fifteen years prior, two soldiers arrive, reporting that Titans are approaching from the South and the West. Despite Angel's protests that they make sure that the explosives in the Titan's mouth succeeded in killing it, Jorge orders a retreat. The captured Titan regenerates, and breaks free from its restraints, and begins devouring one of the soldiers as Angel watches in horror. He is spurred into action by Sorum, who alerts Angel to a nearby Titan. As Angel flees from the Titan, and Sorum fires on the pursuing Titan with his rifle, another soldier arrives to inform Jorge that a ten-meter class Titan is coming from the West.

In the present, Jorge informs Kuklo that the ten-meter Titan that attacked during the expedition was the same one that broke into Shiganshina and devoured Kuklo's mother and Angel's assisstant. Jorge explains that, to distinguish this particular Titans from others, they named it Mammon.