If units are destroyed...
Well, my suggestions would be:
1) Each turn, units replenish based on the territories you control. So if you control territory X, it will supply Y number of unit type W each turn up to a maximum of Z. This means that taking huge losses is not instantly fatal, but neither does it mean that you can bounce back instantly. Possibly this process can be sped up by spending money on developing the territories that provide your preferred troop types.
2) Cards. Apparently, players will get to play cards that will transfer bonuses and stuff in battle. Assuming players can only play a limited number of cards a turn, it would not be inappropriate to have 'healing' cards. Players can then choose to use a card to replenish their numbers, but must then forego some specific battle advantage or something along those lines (because they can only play so many cards in a single turn).
