@rrolo1
They were under the assumption that those were battering rams. Those tend to contain only footsoldier, and not too many. Had heavy cav gotten close to dozens of footsoldiers, either infantry or archers, they might've won, depending on how they go about doing it.
As for the heavy infantry, they think japan is using magic. Magic tends to be exhaustible, meaning they're throwing men until they run out of magic.
As for Japan... They're reliant on two nations to keep them fed, and to keep them with resources needed to keep society and the economy going without problems. So, they're in a way protecting their allies and themselves by invading an enemy to defeat them.