AFAIK that isn't true. Political powers (e.g., feudal lords, kings, etc.) didn't have the resources to conduct warfare on that scale. As examples, centralized government and its resources didn't exist like today, troops had to go home to plant and harvest, and armies depended on local food production in order to survive.

Total war in particular usually involves civilians as targets, and most civilians were far from the battlefield. WWII changed that, when bombers could reach deep beyond the front lines. Without bombers (and missiles), London would have been untouched by the war, for example.

I agree that there's a particularly European tradition of non-total war where maybe you burn the crops and lay seige but let the peasants live because hopefully they'll eventually be your peasants, but the era was more than willing to utterly obliterate schismatics and heretics, and it's pretty obvious that various civilizations have fallen and been obliterated by conquerors of one sort or another throughout history, maybe not as a common norm, but not infrequently. Look at 1 Samuel 15:3 for example, not that I have a good sense of actual historicity of it.

> particularly European tradition of non-total war

What I described is tied to the resources and power of the person in charge. I don't see how that's only European?

This is essentially incorrect. While we can point to legendary incidents such as Carthage in the history of empires, the Mongols are notable in premodern history for systemically using this as a strategy to conquer large rivals by fear and hold a territory size that conventional pre-modern empire building would not allow. Regardless of morality, most empires were better positioned to preserve and exploit life and property than the Mongols. Archeological evidence supports this. This kind of conquest isn’t right either and we aren’t saying all non-Mongols aren’t evil (and I appreciate you were also trying to make this point.)

Total war is a separate concept from genocide. Though both can be practiced in the same war, targeting economic and political support for war does not necessarily mean systemically eliminating a population.

>that conventional pre-modern empire building would not allow I dunno if I'd say this is truly the case. The Mongol Empire was the largest pre-modern empire, yes, but there were other ones spanning millions of square kilometers (multiple pre-horde Chinese dynasties) and several continents (the various caliphates) with these 'conventional' methods.

Most territories back then consisted of people of similar race. An empire attempting to eliminate another could technically be seen as perpetrating genocide. Esp because the treatment of surrendering opponents could go either way.