Uchiha Clan Downfall

The Uchiha Clan Massacre is an event that took place 5 years prior to the Naruto plot. Itachi Uchiha and Madara Uchiha killed their entire clan in a single night using their Mangekyo Sharingans. For previously unknown reasons, they spared Itachi's younger brother, Sasuke Uchiha. Later Itachi states that his reason for killing his entire family was to make Sasuke hate him and want to kill Itachi. This would eventually lead Sasuke to try and gain the Mangekyo Sharingan for power, which is exactly what Itachi wanted. By doing so, Itachi would take Sasuke's Mangekyo Sharingan eyes and replace them with his own failing ones, just like Madara Uchiha did. However, Madara later claims that the reason for this was to protect Sasuke, and that he does so on the orders of the Konoha village leaders (The Third Hokage, Homura Mitokado, Koharu Utatane and Danzo), who had learned that the Uchiha were planning a coup de'tat. The Third Hokage disapproved of the Massacre but the other 3 Elders overruled him.

At this time, the Mangekyo Sharingan is introduced to the Naruto series.