The World They Saw

Karera ga Mita Sekai (彼らが見た世界) is the 3rd chapter of the 19th volume and the 77th chapter overall of the Attack on Titan manga, written and illustrated by Hajime Isayama.

Overview
Bertolt thinks back to when he, Reiner, and Annie were trainees during the recovery of Trost District. He and Reiner contemplate what to do about the situation. Thinking they're alone, Reiner lets it slip that they're Titan Shifters when complaining about the Military trying to plug the hole Bertolt made. Appearing behind them, Marco claims to overhear their conversation. Reiner assures him it was just a joke, but as Marco leaves to help with the recovery, he's tackled by Reiner. Shortly after, Annie appears and Reiner orders her to remove Marco's equipment. The three leave Marco behind in tears and watch in horror as he's eaten by a Titan.

Reiner recalls much later when War Chief Zeke, the Beast Titan, explained to his subordinates that Annie was probably fine. Threatening them, he reminds them of their one goal, to regain the Coordinate and stop this "cursed history." After confirming that the Survey Corps are on their way, the three get into position. Reiner reminds Bertolt to think and act for himself, because he won't be there to tell him what to do. During the battle, Bertolt waits impatiently for Reiner's signal unaware that the soldiers believe they've killed Reiner by blowing his head off. However, his Titan lets out a massive roar, signalling Zeke and Bertolt. Zeke then picks up the barrel containing Bertolt from off the quadrupedal Titan and throws it above the town. The soldiers watch in terror as they wait for the Colossus Titan to burst out and lay waste to Shiganshina.

Summary
During the Battle of Trost District, Reiner and Bertolt comment on the likelihood that Eren will fail in his mission to seal Wall Rose. When Bertolt brings up the possibility of Eren succeeding, Reiner brushes off his worries, pointing out that Eren's success will not matter since they've found what they were looking for. Their conversation is interrupted by the arrival of Marco who, to the horror of Bertolt and Reiner, has overheard their conversation. Disturbed, Marco tries to rationalize that they were merely joking and suggests that they help the Elite Squad fend off Titans. As he heads toward the battle, Marco begins to suspect that the Colossus Titan is a human with the same abilities as Eren, and fails to notice Reiner, who attacks him from behind. Knocking Marco out of the air, Reiner begins to restrain him, saying that he can not let him leave. As Annie arrives, Marco tries to ask her for help but is shocked when she refuses. Bertolt warns of an approaching Titan, and Reiner orders Annie to take Marco's Vertical Maneuvering Equipment. When Annie protests, Reiner reminds her of how she saved Connie's life and demands that she prove that she and her father are still loyal to their cause. As Annie reluctantly takes Marco's equipment, a sobbing Marco begs her to stop. Disposing of Marco's equipment, Annie, Reiner, and Bertolt leave Marco stranded and watch from a nearby building as he is devoured. While Bertolt looks on in horror and Annie begins to sob, Reiner goes into his dissociative state and looks on in horror and says out loud that Marco is being devoured.

Three months afterward, in the early morning before the battle, the warriors are camping atop Wall Maria. Bertolt watches Reiner as War Chief Zeke, the Beast Titan, assures them that Annie is not being tortured. Reminding them that any injury would trigger her Titan Shifting abilities, he suggests that Annie has simply gone into hiding, but Bertolt and Reiner point out that everyone knows her identity. Annoyed, Zeke reminds Reiner of the beating he sustained during their fight, and invites Reiner to fight him again. However, he informs Reiner that if he beats Reiner again, he will force Reiner to give his armor to someone else. When Reiner refuses to fight, Zeke reminds him that the three of them all have a common goal: to obtain the Coordinate, and put an end to their "cursed history". The quadrupedal Titan arrives and informs Zeke that the Survey Corps is approaching. Putting out their camp fire, Reiner and Bertolt begin to go to their positions.

As they run atop the wall, Reiner reminds Bertolt that, as they will be separated, he will have to make his own decisions during the battle. Reiner admits that he has never thought that Bertolt was reliable until recently, and the two agree to finally put an end to everything. As they part ways, the two state their intentions to save Annie, and retrieve Historia Reiss, as they promised Ymir. As he hides inside one of the barrels atop the quadrupedal Titan, Bertolt nervously wonders why Reiner has not yet given him the signal. The soldiers of the Survey Corps enthusiastically celebrate their defeat of Reiner, whose decapitated body now lies exposed in his Titan's nape. Although Jean jeers at Reiner's damaged body, he is shaken by the sight of Connie and Sasha crying, and hurriedly tries to get them to stop. As Hange orders the soldiers to prepare their equipment, Armin attempts to rationalize the Corps' actions to Mikasa. However, Mikasa's attention is drawn to Reiner's Titan form, which has begun to move. To everyone's surprise, Reiner's Titan form begins to roar. As the Corps' soldiers begin to panic, Armin wonders if Reiner is displaying a new ability.

Hearing Reiner's roar, Zeke takes the barrel holding Bertolt from the quadrupedal Titan's back, and throws it towards Shiganshina. As Hange orders the Corps' soldiers to attack Reiner again, Armin notes the possibility that Reiner's roar was a signal for Bertolt to attack. It is at this moment that he sees the barrel containing Bertolt flying through the air, and frantically informs Hange that Bertolt will be attacking the soldiers from above.