Weapons of mass destruction would/will be the main weapons of world war 3, petty differences between the skill of various infantry units are not important. Of course modernised loyal infantry are a necessity in maintaining whatever is left of a country after a nuclear exchange, however what is left of a country is more of an issue.
I think the nuclear powers will blast each other into obscurity, leaving Australia , Brazil and Argentina as the most powerful economic and military nations. Though this may mean they will be a target along with the nuclear powers. China, India and the middle east will be targets and experience dehabilitating environmental disasters and any current military powers there will collapse. Europe, eestern Russia, the mediteranean and North America will be sterilised by the radiation and chemical weapons, the radiation will wear off in 2 years or so, the much smaller areas affected by chemical weapons may experience permanent damage.
A few areas and bunkers in Europe and North America aswell as eastern and northern russia will still be habitable, though won't be able to produce much food and fall out will be a problem for 6 months after the nuclear exchange. People in these ares will have to move out after living off their tins of baked beans for how long they've been hidden away.
Unstable regions of the world would experience apocalyptic famine and no ozone layer, obviously they will be thrown into civil war and ethnic conflict. Eventually the ozone layer will reform. 10 years after the disaster and most of the military powers will be living off very crude fuel and ammunition industries supported by early 20th century style agriculture. Very little pre-apocalypse social structures will be left standing, just the remnants of the military forces that survived and those who claim to be a continuation from that wonderful pre-apocalyptic world.