Get pirates on your side. The battle of Itsukushima

Hatsukaichi-city, Hiroshima A.D.1555
Mōri Motonari was the one who pushed up the Mōri family that was merely a Jisamurai lord in Aki province (Western Hiroshima) up to a supreme ruler in Chugoku region. In the battle of Itsukushima, he lured Sue Harukata to Itsukushima who raised a rebellion against Lord Ōuchi Yoshitaka and became a major force; Mōri cornered Sue with a select few. Sue went to Itsukushima with 500 navy boats and more than 20,000 troops. On the other hand, Mōri who has set the trap had 120 navy boats and 4,000 troops. Although the Sue army should have overwhelmed Mōri in numbers, the Murakami navy pirates of Setouch sent reinforcements right before the battle began, and they landed with a sneak attack in the teeth of a ferocious storm; the waterway was blocked, they were pushed to the deep west of the island, and Sue Harukata committed suicide by the sword. In this battle alone, Sue army was said to have had 4,700 casualties. After the battle, Mōri had the blood on the Itsukushima Shinto shrine cleansed, had the bloody soil in Itsukushima shaven away, and completely made it a taboo to have the impurity of blood within the sacred area of the Shinto shrine.
Highlights of the map
It was impossible to beat the Sue army with an overwhelming number of troops by repeated field battles with few troops. It was a matter of time until the Mōri Castle in the Northeast of Itsukushima was going to be invaded. To avoid land warfare at all cost, Mōri aimed for the strategy to drive them into the island. They planted rumors to lure Sue Harukata into Itsukushima. Despite the warning from his senior vassal who saw through the rumor, Sue blindly believed Mōri’s rumor and went to the island with a large army of 20,000. That sealed his fate.
Behind the scenes drama
Mōri Motonari is said to have made a name for himself with the support of his three sons. In this battle of Itsukushima, his second son Kikkawa Motoharu was on his side, and his third son Kobayakawa Takakage supported him in the back. The two sons played active roles. On the other hand, his heir Ryugen was held hostage in Ouchi family that was the lord of Sue, so he solidified the expansion of his father’s power from outside of the battlefield with his domestic affairs skills afterward.