The 1555 was the only battle to be fought on the sacred island of Miyajima; the entire island is considered to be a Shinto shrine, and no birth or death is allowed on the island. Extensive purification rituals took place after the battle, to cleanse the shrine and the island of the pollution of death.
The Battle of Miyajima was the turning point in a campaign for control of the Ōuchi clan and of Aki Province, a strategically important province for establishing control of western Honshu. It was an important step for the Mōri clan in taking the foremost position in western Japan, and cemented the reputation of Mōri Motonari as a cunning strategist.
Background
In 1551, Sue Harukata revolted against his lord Ōuchi Yoshitaka in the Tainei-ji incident, forcing him to commit seppuku. Sue installed the next lord of the clan, Ōuchi Yoshinaga (younger brother of Ōtomo Sōrin), but effectively led the Ōuchi family and its armies, intent on military expansion.
In 1554, Mōri Motonari, as a vassal of the Ōuchi clan, wanted to avenge the betrayed Yoshitaka, and so he rebelled against Sue, whose territorial ambitions were depleting clan resources. The heavily outnumbered force under Mōri attacked and defeated Sue at the Battle of Oshikibata. Mōri then departed from the mainland to build a fort, known as Miyao Castle, on Miyajima while proclaiming publicly his woe that it would not hold out long against an attack.
Battle
Miyao Castle was built on a hill near Itsukushima Shrine and facing the mainland, making it a visible and tempting target. Sue commandeered a fleet of merchant vessels and prepared the troops of the Ōuchi clan to cross the channel. In the early hours of 15 October, Sue attacked Miyao Castle in an amphibious frontal assault.
With an embarkation point secured, Mōri Motonari continued with his elaborate plan. He had enlisted the aid of the Noshima Murakami, a gang of local pirates led by Murakami Takeyoshi, who agreed to transport the Mōri troops to Miyajima. Sue Harukata escaped from the confines of Miyao Castle, but when he saw that escape from the island was not possible, he also committed suicide by seppuku. The Ōuchi clan's senior vassal Hironaka Takakane fought, together with his son, to the end, and he was also killed.
1000px|thumb|center|Scroll depicting the invasion by Mōri forces (ca. 1855)
Aftermath
The Sue forces at Miyajima are estimated to have been about 20,000 to 30,000 men, and though estimates of the combined forces under Mōri Motonari range widely from 4,000 to 10,000 troops,