Nagashima (長島) was a series of fortresses and fortifications controlled by the Ikkō-ikki, a sect of warrior monks in Japan's Sengoku period who opposed samurai rule.
This, combined with the surrender of the Ikki's other main fortress, Ishiyama Hongan-ji, several years later, ended the threat the Ikko-ikki posed to Nobunaga and other samurai conquerors.
The fortress was situated on a swampy delta, on the border of Owari and Ise Provinces, at the point where three rivers converge, to the southwest of the modern-day city of Nagoya.
The Oda clan, which controlled nearby lands, was wary of the Ikki's growing power, and engaged them at Ogie Castle in 1569.
Nobunaga returned to lay siege to the Ikki's fortress three times, before finally subduing it in 1574 with the help of the Kumano Suigun (Navy) under the control of the Kuki family.