I don't think the presence of magical healing is going to have a huge impact on war, actually. you figure most of your troops (conscripted commoners) are going to have few hit points, on average, than damage dealt -- so healing isn't going to fix them. They are already dead. For higher level leaders, elite troops, etc... it is a matter of distribution of resources. The army that loads up on healers is going to be light on offense and therefore going to get massacred. Eventually, a balance will occur where, while what large scale conflicst in a D&D world would look like doesn't resemble 'normal', the end results will be the same.
The thing that causes problems with D&D war is that death is the only long standing result of injury. Even a greivously injured (-9 hp) commoner is going to be back on his feet in a week and a half. No infection. No crippling. Not even a twisted ankle. I am not myself a fan of adding this kind of thing to D&D, as it bogs the game down, but the presence of secondary injury effects would go a long way in making the world seem more 'realistic'.
Really, this is an example of having to ask yourself whether the rules of the game serve as the laws of nature in the world. If the rules are just an abstraction to facilitate play and let everyone have fun, then the impact on 'realism' is reduced. NPCs and peasants do get sick and do suffer long term effects from massive injury. PCs just happen to be too cool for that. If, however, the world really works this way, and people don't suffer from such effects unless it is caused by something else, or don't react to injury until near death, everything starts to change. Throw in the hit point bloat, healing magic, and increasing saves (particularly versus mundane diseases and toxins) and even non adventurers are supermen. i like option 1, but the second could be used to create a very interesting, very different, and ultimately very cool world.