Attack on Titan

Humanity hides behind three concentric walls built taller than any of the giant humanoid Titans that have driven the species to near-extinction. Inside the walls, life has been peaceful for a hundred years. The manga opens on the day a fifteen-meter colossal Titan kicks a hole in the outermost ring and lets the smaller Titans pour in. Among the survivors are Eren Yeager, his foster sister Mikasa, and his best friend Armin, three teenagers who enlist in the military's Survey Corps when the rebuild is complete. The story spans their training, the truth behind the walls, and the war behind the truth.