From what I remember reading, Sherman's March to the Sea had general orders to destroy Southern economic output but not to wantonly harm civilian population or things necessary to keep them alive, although even if true, it's an open question on what level of discipline was maintained over the campaign at the unit level.
The Romans largely set the gold standard for total warfare, often annihilating their opponents armies completely, burning their cities to the ground, looting everything of value and enslaving anyone left. Carthage and Gaul come to mind. Marcus Licinius Crassus had 6,000 rebel slaves crucified on a stretch of the Appian Way miles long to serve as a warning to any continuation of the rebellion.
The thing is, in modern military campaigns I don't think you would have to actually destroy an entire country completely or kill all their civilians. My sense is that after a brief period of time where you had firmly established total warfare as the core strategy you would cow the population. A path through Iraq or Syria wide enough for a couple of divisions of mechanized infantry where every form of resistance was met with total destruction would result in a quick calculus that resistance really was futile and that subjugation was a better choice.