Uchiha Clan Downfall

Uchiha clan vs. Itachi Uchiha Winner: Itachi Combatants: Uchiha Military Police Force     vs.     Itachi Uchiha Uchiha Jonin Uchiha Chunin Uchiha Genin Casualties Every member of the Uchiha               None clan, except for Sasuke

Summary
Over time Itachi began to distance himself from the village and his family,[39] eventually becoming a prime suspect in his best friend, Shisui Uchiha's, death.[40] Before the death could be properly investigated, Itachi killed all the members of the Uchiha clan in a single night so that he could "test [his] ability".[41] Itachi decided to spare Sasuke, however, feeling he was not worth killing;[42] instead, prior to leaving, Itachi told Sasuke to live hating him and get strong enough to avenge the clan. At some point in time Itachi joined Akatsuki. While it stands to reason that he joined after massacring the Uchiha clan, the timeline that has been given throughout the series coupled with his age suggests that he joined the organization prior to killing his clan.