Nobunaga faced a significant threat from the "Ikkō-ikki", a resistance movement centered around the Jōdo Shinshū sect of Buddhism. The "Ikkō-ikki" began as a religious association for self-defence, but popular antipathy against the samurai from the constant violence of the Sengoku period caused their numbers to swell. By the time of Nobunaga's rise to power, the "Ikkō-ikki" was a major organized armed force opposed to samurai rule in Japan. In August 1570, the Ishiyama Hongan-ji War begins, Nobunaga's campaign against the "Ikkō-ikki", while fighting against his samurai rivals. In May 1571, Nobunaga besieged Nagashima, a series of "Ikkō-ikki" fortifications in Owari Province, beginning the Sieges of Nagashima.