Sekigahara is one of the most famous battles in Japanese history.

It took place in 1600.

It’s remembered for a stunningly creative piece of tactical thinking.

Two armies, of 80,000 men each, confronted each other.

Ieyasu commanded the army of the east.

Mitusnari commanded the army of the west.

A third army, commanded by Hideyaki, waited on a hill overlooking the battle.

Hideyaki was supposedly an ally of Mitsunari.

But he’d actually made a secret deal with Ieyasu.

During the battle he would switch sides and help Ieyasu win.

The two main armies fought.

Hideyaki’s army sat on the hill and watched.

The battle between Ieyasu’s army and Mitsunari’s troops was bloody.

Still Hideyaki just sat and watched.

Ieyasu sent messenger after messenger asking him to fight.

But Hideyaki didn’t move his troops.

He couldn’t decide what to do.

So his army still sat there and watched.

The situation was going badly.

Unless Hideyaki came into the battle, as he’d promised, Ieyasu would eventually lose.

Ieyasu sent increasingly desperate messages to Hideyaki.

But Hideyaki didn’t move.

He seemed paralysed with indecision.

So Ieyasu did what is remembered as a brilliantly creative manoeuvre.

He turned his guns onto Hideyaki’s army.

10,000 troops began firing into the massed ranks on the hill above.

As his troops began dying, Hideyaki was shocked out of his indecision.

He ordered his troops to charge down the hill on Ieyasu’s side.

And his army did indeed make the difference.

Combined with Ieyasu’s troops, they defeated Mitsunari’s forces.

The two victorious armies took 60,000 heads that day.

Ieyasu went on to become Shogun.

Sole ruler of all of Japan.

All because, in the middle of battle, he did what no one else would have thought of.

He fired on his own ally, and shocked him out of his indecision.

To realise how brilliant that was, we have to unpack the thinking.

Ieyasu saw that Hideyaki was paralysed by having too many choices.

Hideyake was thinking, do I:

a) stay where I am until the battle’s over, or

b) come in on Mitsunari’s side, or

c) come in on Ieyasu’s side, or

d) wait and see how things develop, or

e) think of another option?

Meanwhile, while he’s thinking, he’s not committing.

And Ieyasu is losing the battle.

Ieyasu realised that without Hideyaki he would be beaten.

Therefore he had nothing to lose.

He turned his guns on Hideyaki.

Thereby saying: even if I Iose, unless you come in on my side right now, I will destroy you.

This gave Hideyaki a simple binary choice:

a) I attack on Ieyasu’s side and win, or

b) I stay where I am and lose my entire army.

And, of course, a choice like that is really no choice at all.

So Hideyaki was forced to act.

Ieyasu set up the question so that the only possible answer was the outcome he wanted.

Which is, of course, exactly what choice architecture is all about.

Don’t offer a range of solutions.

Reduce the choice to two options.

One of which is hugely preferable.