The Stohess District raid was a conflict between the Survey Corps and the Female Titan within Stohess District, the eastern city of Wall Sheena, in which the Female Titan was captured and the existence of Titans within the Wall was discovered.
Prelude: During the 57th Expedition Outside the Walls, Armin Arlert encountered Annie Leonhart, the Female Titan, and was able to deduce her identity after considering her actions during the expedition and seeing her Titan's face up close. After the Survey Corps' return to Karanes District, he shared this deduction with Commander Erwin Smith, who devised a plan to capture Annie in Stohess District, Annie's base of operations in the Military Police Brigade and the half-way point between Karanes and Mitras where the Survey Corps had been summoned to hand Eren Yeager over to the MPs. This plan was shared with Eren, Mikasa, and Captain Levi first, though Eren was in great disbelief and denial at the possibility that Annie was the Female Titan.
The raid: On the morning of the raid, the new recruits of the Military Police Brigade, including Annie Leonhart, were assigned with escorting the Survey Corps through Stohess District on their way to Mitras. As Annie joined her comrades on their way to the escort mission, she was beckoned aside by Armin who informed her of a plan by the Survey Corps to sneak Eren out of the district. After some convincing, Annie agreed to help Armin and the others.
First encounter: On the morning of the raid, the new recruits of the Military Police Brigade, including Annie Leonhart, were assigned with escorting the Survey Corps through Stohess District on their way to Mitras. As Annie joined her comrades on their way to the escort mission, she was beckoned aside by Armin who informed her of a plan by the Survey Corps to sneak Eren out of the district. After some convincing, Annie agreed to help Armin and the others.
Titan battle: In his Titan form, Eren engaged in combat with Annie in the streets of Stohess District. Upon hearing the Titans' brawl elsewhere in the city, the Survey Corps' escort came to a halt as Commander Nile Dok sent his recruits to investigate. Commander Erwin Smith advised him to dispatch all available Military Police officers, and the escort to Mitras ceased.
Source: Stohess District raid, available under CC BY-SA 3.0.