Air Nomad Genocide (Earth-1529)

"With Roku gone, and the great comet returning, the timing was perfect to change the world. Fire Lord Sozin knew the next Avatar would be born an Air Nomad, so he, with the First Hokage's help, wiped out the air temples. Unknown to the Fire Lord, the First Hokage kept the surviving Air Nomad lineage hidden in Konoha. Sozin wasted the remainder of his life searching in vain, as the Second Shinobi World War waged on. The greatest hope of the shinobi world... the last airbender."

- - Third Hokage Hiruzen Sarutobi narrating the memoirs of the genocide

The Air Nomad Genocide was an enormous massacre carried out by the Land of Fire and Konohagakure against the Air Nomad clan, nearly resulting in the extinction of the entire clan, as well as the fauna that lived in their air temples. Among the very few survivors of the massacre were the family of Kushina Uzumaki, mother of the future Avatar Naruto Uzumaki, which were ironically the prime target. The outrage over this destruction of an entire culture led the other four of the five great nations to declare war against the Land of Fire, leading to the Second Shinobi World War.