"War is hell." Why do think this expression came to be? Invading armies lived of the land. That is, they pillaged what they needed from the enemy. Two birds with one stroke: every cartload taken from the enemy was a cartload less for the enemy and multiple cartloads that didn't have to come from the motherland. Even Sun Tzu recommends to pillage of the enemy for that reason.
The villagers were left with little or nothing at all. How (and/or: if) they survived wasn't a problem for the invading army. As far as I know there were very little rules with regard to warfare, pillaging or the treatment of prisoners. Some rules were adhered to (ransoming rich POW's) others much less so (churches and temples were often looted anyway).
'The environment' is a fairly new concept. It didn't exist in ancient times. The effect of ancient (Roman, Hunnic and other large armies) were close to zero on the environment. The burning and pillaging didn't last long enough to affect the environment, and the depopulation was (from an environmental point of view) only beneficial for the environment.
Care for wildlife was completely absent in ancient times. The Roman army had hunting on its peacetime roster, to supplement the legionary meal. In ancient times meat was expensive and anything with legs was edible. This was on campaign of much more importance. A few roasted sparrows or slice of badger can liven up your hard tack considerably.
With regard to Napoleon: his logistics were totally incapable of supplying the army, long before Moscow. Even if he had taken the coastal route and took St. Petersburg, where his army could be supplied by sea, that was totally insufficient either. By land towards Moscow: too silly to even speak of. Which makes me wonder why a) he decided to invade anyway and b) why he was/is considered one of the greatest generals of all time. The entire Russian campaign isn't a credit to the man. But that is a very different question.