Yeah see the way I know this isn't what happened is that Yomi started surrounded by throne provinces and never managed to expand past its capital. So it never won a battle, probably because it STUCK A FUCKING SLAVE COLLAR ON ITS PRETENDER 
No, no... that's not a guarantee. Events can cause battles, too, and while as far as I can recall they're incredibly unlikely (save for the troll) to generate anything with an item, I'm pretty sure it's still
possible. If collars really can be picked up post fight, there's still a(n incredibly small) chance Yomi could have basically landed on just about the worst RNG results in the history of D4, instead of making a rather impressively bad decision.
... it's bloody close to certain it was the AI being an idiot, but... it
might not have been. The possibility may be there (assuming slave collars actually can be picked up*).
*Which if they can, I'm totally going to have to remember collar bombing as a tactic if I ever run a blood nation. Mass produce scouts and collars, send them into a vital enemy army to die and cripple important stuff~