The Ishiyama Hongan-ji War (石山合戦, Ishiyama Kassen) was a ten-year military campaign that took place from 1570 to 1580 in Sengoku period Japan, carried out by lord Oda Nobunaga against a network of fortifications, temples, and communities belonging to the Ikkō-ikki, a powerful faction of Jōdo Shinshū Buddhist monks and peasants opposed to the rule of the samurai class.

Oda and Tokugawa had fought the Ikki before, crushing their armies of Mikawa Province and other areas, and by 1570, their twin fortresses of Ishiyama Hongan-ji and Nagashima were their last bastions of strength.

This was no easy task, however, because the Ishiyama fortress sat on the coast, which was guarded by the fleet of the Mōri clan, masters of naval combat and Oda's enemies.

By early 1575, however, the fortress was already in urgent need of supplies, and the Abbot Kōsa was ready to begin peaceful overtures with Nobunaga to end the siege.

He led a number of ashigaru (foot soldiers) army of only 3,000 men to attack as many as 15,000 enemy forces, along with Niwa Nagahide, Hashiba Hideyoshi, Takigawa Kazumasu, Hachiya Yoritaka and Inaba Yoshimichi.

But Nobumori made no progress against the Hongan-ji forces and in the meantime Kuki Yoshitaka failed to blockade Mōri supply lines in the first Battle of Kizugawaguchi.

[4]: 289 Nobunaga ordered Shibata Katsuie to conquer the Hokuriku region, sent Takigawa Kazumasu to suppress ikko-ikki rebels at Kii province, Hashiba Hideyoshi to conquer the Chūgoku region from the Mori clan, and Akechi Mitsuhide to pacify Tanba Province in 1577, and Nobunaga eventually blocked the Mōri's supply lines.

[3]: 229 [4]: 290 With respect to Imperial order, Nobunaga spared the lives of many of the defenders, including Shimozuma Nakayuki, but burned the fortress to the ground.