20 : Erwin Smith: 57th Expedition Beyond The Wa... Now

The expedition is ultimately called off as a failure due to the high casualties and the escape of the spy. The episode ends on a tragic note as the cloaked mole, still hidden among the Scouts, ambushes Levi’s squad and kills Gunther Schultz, setting the stage for the devastating battles to follow. 57th Expedition Outside the Walls | Attack on Titan Wiki

Only veteran Scouts who had survived for five years or more were briefed on this true mission to ensure that any potential mole was kept in the dark. A Game of Wits: Capture and Counter-Strike 20 : Erwin Smith: 57th Expedition Beyond the Wa...

Erwin intentionally used Eren Yeager as bait to lure an intelligent Titan—later revealed as the Female Titan—into a trap. The expedition is ultimately called off as a

While the official goal of the 57th Expedition was to secure a route to Shiganshina, Erwin’s true motive was a high-risk gamble to flush out a spy hiding within the military ranks. A Game of Wits: Capture and Counter-Strike Erwin

Despite being pinned, the Female Titan hardens her skin, preventing Levi and Miche from cutting out the human pilot.

This episode highlights the brutal utilitarianism that defines Erwin's leadership. As Armin Arlert famously notes during the mission:

Erwin's willingness to sacrifice hundreds of soldiers for a chance at capturing the enemy proves that for him, the only way to defeat monsters is to rise above humanity's limitations. Aftermath and the Cost of Failure