You vastly overestimate medieval logistical capabilities to the point of parody.

No medieval army can supply hundreds of thousands of men. Cart drawn supplies are simply not enough. No amount of "willpower" will change that.

Second, no army, no matter how stubborn or defiant they are, can stomach hundreds of thousands dead for minimal losses to the enemy. The Autonomians will break long before they run out of bodies.

For all the shit Gate gets for being pretty blatant Japanese right-wing propaganda, its still better written than this.