The biggest problem with such an approach is that it can be difficult to prepare for. Whenever you have such a broad theme as "conquer a country" the PCs can go about it in so many ways it can be very difficult to maintain a unified plot line. If you're good at improvising though I'm sure it would work fine.
Heroes of Battle is great, but its more of a "Saving Private Ryan" approach to D&D. It works great for putting PCs in mass-combat situations, but actually have the PCs guide the strings? Dunno.
From my experience, the players were much more interested in awesome big combat scenes rather than resource management.
I haven't had much success going from adventures -> soldiers, but I have had a lot of luck with soldiers -> adventurers. Think Joss Whedon's Firefly.
Dungeons are essentially bunkers, and aren't too difficult to implement.