Oda Nobunaga was born in 1534 into a country at war with itself. Once ruled by emperors and later by shoguns, Japan was by then split into fiefdoms whose samurai fought endless battles with one another. Inheriting a fief in central Japan, Oda spent 30 years trying to put the situation right. He was not an altruist, a pacifist or a diplomat. There was only one way of unifying Japan, he thought, and he made of it a motto: “Rule the realm by force.”

In battle after battle, Oda proved to be a first-class tactician. He defeated far larger forces than his own using surprise attacks. On land, he had his engineers widen roads and build pontoon bridges ahead of him to accommodate the rapid movement of enormous numbers of troops. On sea, he was the first in Japan – perhaps in the world – to send iron-clad ships against his enemies.

Oda changed forever the sound of battle in Japan, becoming the first leader to make systematic use of the firearms brought to the country by Portuguese traders in the 1540s. Whereas proud samurai fought from horseback, using swords and bows that took long years to master, Oda trained peasant foot soldiers to fire muskets in rotating ranks. Load, point, shoot, repeat: simple, yet so effective in bringing down a cavalry charge that samurai began seeking out pock-marked armour – when it came to stopping a bullet, they preferred battlefield proof to a forger’s promise.

In this way, Oda dispensed with secular and religious enemies, the latter taking the form of Buddhist sects boasting their own armies, rural guerrillas and towering fortresses. Oda used fire, starvation and mass execution against monks and laypeople alike – to impressive strategic and psychological effect, but also to satisfy a more personal lust. At one New Year banquet he threw, the centrepieces were the skullcaps of two enemy warlords lacquered in silver and gold for use as sake cups; these grisly vessels were passed around while a celebratory song was sung.

In 1568, Oda fought his way into Japan’s capital city, Kyoto. There he bent a powerless shogun to his will and used a penniless imperial family to provide fig-leaf legitimacy for a dictatorship that covered the heartlands of Japan’s main island of Honshū. As his grip tightened, Oda turned warlord after warlord effectively into his vassal, and began the process of restoring trade, agriculture and taxation in support of his newly united nation.

On his way west in 1582, preparing to claim yet more of Japan for himself, Oda was surrounded in a temple by allies-turned-traitors who attacked him. He died as many of his victims had: in pain and confusion, engulfed in flames. But his project lived on. A humble foot soldier called Toyotomi Hideyoshi had risen through the ranks to become a trusted general. He picked up where his boss left off, with an Oda ally, Tokugawa Ieyasu, completing the job in 1600 and bringing to Japan a peace that lasted well into the 19th century.

Japanese schoolchildren learn about these men, Oda foremost among them, as the ‘three unifiers’, using a little ditty to tell a great tale: Nobunaga pounded the rice.

The sun descending in the west,
The evening star does shine;
The birds are silent in their nest.
And I must seek for mine. 
The moon, like a flower
In heaven’s high bower,
With silent delight
Sits and smiles on the night.
Farewell, green fields and happy grove,
Where flocks have took delight:
Where lambs have nibbled, silent move
The feet of angels bright;
Unseen they pour blessing
And joy without ceasing
On each bud and blossom,
And each sleeping bosom.
They look in every thoughtless nest
Where birds are covered warm;
They visit caves of every beast,
To keep them all from harm:
If they see any weeping
That should have been sleeping,
They pour sleep on their head,
And sit down by their bed.