The Tragedy of Yosuga Pass.
The Tragedy of Yosuga Pass (ヨスガ峠の悲劇, Yosuka Tōge no Higeki) is an event that happened ten years prior to the start of the Fourth Shinobi World War.
Background
Three shinobi from Kirigakure, Suiren, Ganryū and Junsai were sent on a mission to retrieve a document taken by Kumogakure with Iwagakure's assistance. However, when the document was retrieved, the Iwa shinobi sent betrayed the Kiri team and killed Junsai and Suiren, who sacrificed herself to save Ganryū at Yosuga Pass. Since that day, Ganryū and other Kiri ninja who disapproved the betrayal held a deep hatred against Iwa. This incident, known as the Tragedy of Yosuga Pass marked the beginning of hostility between Kirigakure and Iwagakure. moooooooooooo