Delusions of Strength

Buryoku Gensō (武力幻想) is the 4th chapter in the 4th volume and the 17th chapter overall of the Attack on Titan manga, written and illustrated by Hajime Isayama.

Overview
Eren and Reiner are practicing hand to hand combat when they see Annie Leonhart slacking off and Reiner volunteer's Eren to challenge her. After they duel and she easily defeats both of them, Eren asks her where she learned to fight. She states that all of this is pointless, and gives him a closer look inside the military system. Later on in the mess hall, Eren hears Jean Kirstein bragging about his skills. Annoyed, this causes Eren to fight with him and flips Jean with Annie's technique when he grabs Eren's shirt, lecturing him and saying he is a coward only joining the Military Police Brigade for safety, catching Annie's attention. When the sound of the commotion attracted the instructor, Mikasa passed it off as Sasha Blouse passing gas, much to her embarrassment. The next day in the training grounds, Annie and Eren notice Jean working harder. When Eren asks Annie for her opinion on his kicks, Annie dismays them and offers to teach Eren. When he turns her down she tells him he does not need to hold back and Eren agrees to the lessons.

Summary
Two years since the start of their training, in the year 849, the trainees are now practicing hand to hand combat. Eren and Reiner are sparring together. Reiner charges at Eren with a wooden knife and Eren flips him almost effortlessly. Surprised, Reiner asks how he is able to do that. Eren states that he's had practice with a local big kid he used to play with when he was younger. Eren also voices his concern about practicing fighting humans. Reiner then says that it's better to be prepared.

As they finish their conversation, they both see Annie Leonhart who is slacking again and avoiding the instructor. The two confront her and Eren ends up trying to practice with the wooden knife on her. As Eren rushes, she sidesteps and trips Eren, who lands face first in the dirt. Going for another round, Annie grabs Eren's arm and disarms the knife. Tossing the knife to Reiner, she tells him it's his turn. Hesitant, Eren reminds him that it's a soldiers responsibility. Reiner charges and Annie takes him down easier than she did Eren. Curious, Eren asks her who taught her to fight. She replies that her father did. Annie agrees with Eren about hand to hand combat, thinking that it serves no purpose. Using other trainees as examples, she tell Eren that their score depends mostly on their aptitude with Vertical Maneuvering Equipment, the rest just about claiming privileges inland. Taking the wooden knife, she runs at Eren and easily gets past his guard. Eren attempts to flip her as he did Reiner, but she reverses it and pins him to the ground with the knife at his throat. She reveals that her father made her learn these moves and the she no longer derives any enjoyment from playing soldier, with that she drops the knife and walks away.

In the mess hall, while Armin talks at length about the nature of the Military Police Brigade privilege, Eren overhears Jean bragging about his techniques. Annoyed, Eren asks Jean why he is honing his Titan killing skills all in an effort to get far away from them. The argument seems to be dying down until Jean sees Mikasa. After that, he grabs Eren's collar, causing the situation to escalate again. Remembering Jean's attitude was similar to his own, Eren uses the technique that he picked up from Annie, which catches both him and Annie by surprise. After hitting the ground, he is angry with Eren for what he did and confronts him for that action. He then confronts Jean for his behavior and doesn't think he's a soldier, but a coward for wanting to join the Military Police just to be safe from the Titans. The sound of Jean hitting the ground causes the instructor to come to the mess hall. Fortunately, they are able to get back in their seats before he sees the reason for the noise. Asking what happened, Mikasa answers that it was just Sasha passing gas, much to Sasha's dismay. The instructor tell her to show some self-restraint, causing her even more embarrassment. Eren comments that, had they been caught, it could have cost Jean his opportunity to join the Military Police. Jean replies that he fears their score will never be settled. Eren suggests they fight during combat training so it can be passed off as innocent sparring.

The next day, Eren and Annie notice that Jean is working unusually hard. Annie tells him this is all in an effort to overcome him and doesn't see anything positive out of it. For a brief moment, Annie thinks back on the days when her father trained her to fight in her hometown. Eren asks about his form when he mimicked her throw. Annie replies that "it sucked", but since he likes the move she offers to teach him. Not wanting to be beaten up by her again, Eren turns her offer down. She offers again, and tells him that he doesn't have to hold back. Eren agrees and Annie teaches him the technique properly with a firsthand demonstration.