Hyūga Affair

Nine years prior to the start of the series, Kumogakure and Konohagakure were at war. To end it, they agreed to a peace treaty on the day of Hinata Hyūga's third birthday. The treaty was soon revealed to be nothing more than a cover when the Head Ninja of Kumogakure at the time, who was in Konoha to sign the treaty, attempted to kidnap Hinata in order to obtain the Byakugan and its secrets. The Head Ninja was killed in his attempt by Hinata's father, Hiashi Hyūga.

Kumogakure denied all accusations, and demanded the dead body of Hiashi as compensation for the death of their Head Ninja. At the request of Hiashi's twin, Hizashi Hyūga, as well as the Hyūga clan and the Third Hokage, who didn't want to lose the heir to the clan, decided to send Hizashi's body instead. This had another advantage, besides saving the life of the Hyūga heir; Hizashi's Cursed Seal would seal his Byakugan after his death, safeguarding its secrets.

The details surrounding Hizashi's death are quite well-known among the high-ranking ninja of Konohagakure, most of them taking it as a serious infraction of Kumogakure. This was shown when Yamato called out the Fourth Raikage for his relentless pursuit of Sasuke Uchiha, comparing the latter's supposed theft of Killer Bee to their attempted theft of the Byakugan and noting that Konohagakure remained magnanimous while they were vindictive. While they are not allies, Konohagakure and Kumogakure seem to have come to friendly terms. Nowadays, this incident is known as the in Kumogakure.