Could you mean...
"the Battle of Yakshagahara*"
perhaps!?
[Continuing from the gorgeous color pages!!]
*Yaksha are Buddhist guardian spirits often depicted as demonic warriors, "gahara" means "Field of"
Oh yes, I think that's what it's called there.

"The Battle of Yakshagahara"?
What's that?

A war the shogunate had against the outside world,
which ended with them suffering a major blow.

The Order of Cadoract ordered multiple nations to form an allied front
and begin a war against the Land of the Rising Sun,
but they were overwhelmed by the lionhearted Samurai
until the battle became an one-sided affair.