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.
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, 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.
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. Meanwhile, Mōri took advantage of his absence to seize Sakurao Castle, Sue's castle on the mainland.
With an embarkation point secured, Mōri Motonari continued with his elaborate plan. He had enlisted the aid of local pirates who agreed to transport his troops to Miyajima. The fleet carrying the Mōri forces set out in a driving thunderstorm. Their approach thus obscured, Motonari and two of his sons, Kikkawa Motoharu and Mōri Takamoto, landed on the east side of the island, to the rear of the Sue force. Meanwhile, Motonari's third son, Kobayakawa Takakage, sailed straight toward Miyao Castle in a feint, then retreated so he could be in a position to return the following day, his attack synchronized with the overland assault. At dawn, Takakage and his 1,500 troops landed before the small fortress, and the sound of shell trumpets signalled that all units were in position and the attack commenced. As Takakage's force rushed the front gate of Miyao Castle, Mōri and his troops hit the Ōuchi position from behind. Caught completely by surprise, many of the Ōuchi troops scattered in disarray. Hundreds tried to swim to the mainland and drowned in the attempt. Many more saw that defeat was inevitable and committed seppuku. By 18 October 1555, resistance had ended at a cost of about 4,700 dead among the Ōuchi army. 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.