Yes, but the crusades were a rather different setting, where the Christian world would unite against the muslims (for, really, more political reasons than "retaking the Holy City"). However, I was referring more to smaller wars, sometimes even internal, between two disputed counts or smaller entities. Looting occurred. Maybe I expressed myself a bit wrong. It wasn't /that/ common after all, but it certainly happened.
I think depending on what period you look at, merchants were more and more common for food supply.
I think you underestimate the piety of the medieval world. Except for the Normans. They were totally in it for the lands.
But yeah, in small wars between counts and stuff you'd be less likely to keep a train of merchants simply because you'd travel a few dozen miles, commandeer peoples houses and food, bring a bit of your own, maybe hunt if you're lucky, then fight. Or you'd already be looting since thats what most war boiled down to. In wars between kingdoms you'd be a lot more likely to work with merchants than to loot them simply because a large concentration of men and horses can very quickly eat everything in a small area.
