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Itachi claimed that he did it as part of a plot to obtain the Eternal Mangekyou Sharingan by killing the Clan (gaining his MS) and forcing Sasuke to grow strong and gain his own MS, so Itachi could kill Sasuke and take Sasuke's MS, thus obtaining the EMS.
Sasuke realised that Itachi wasn't powerful enough do kill the entire Clan without help, and figured out that Madara helped Itachi. Itachi admitted this.
Madara told Sasuke that Itachi was ordered to kill the Uchiha, but couldn't bring himself to kill Sasuke, and so contrived a plot in which Sasuke would grow strong, then Itachi would die at Sasuke's hands, making him a hero to Konoha, so the Elders couldn't kill him even after Itachi died.
Your spoiler was a bit off.
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18-Feb-2011 00:19:38