Tragedy of Yosuga Pass

The 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 immediately betrayed the Kiri team on the orders of their Tsuchikage. This resulted in Junsai's death and Suiren sacrificing herself in order to save Ganryū at Yosuga Pass. Since that day, Ganryū and other Kiri ninja who disapproved the betrayal had grown to despise Iwa and its shinobi. This incident, known as the Tragedy of Yosuga Pass marked the beginning of hostility between Kirigakure and Iwagakure.

Near the beginning of the Fourth Shinobi World War, the Third Tsuchikage sent an envoy to formally apologise for the incident, sending the Kiri ninja the personal belongings collected from Kiri's fallen shinobi.