I think that the best way to improve the combat would be addition of a retreat mechanic.
For example, if you have 2,000 units defending, and are attacked by 1,900 units, then the enemy army will be annihilated, and the defender will be almost annihilated.
In real life, the attacker would probably attack, lose dome amount of units, and realize that they are losing, so they would retreat. So the 2,000 units would lose like 400 units, while the attackers would lose 1000. This would leave the attacked less exposed to all out conquest, while still giving the defender initiative to attack. It would also solve the issue of a country being conquered by micro-armies.