The Tragedy of Yosuga Pass.
Ten years prior to the Fourth Shinobi World War, three shinobi from Kirigakure, Ganryū, Suiren 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. Since that day, Ganryū and other Kiri ninja who disapproved the betrayal held a deep hatred against Iwa.