I don't understand why 2 armies?
When the country is conquered, the rules say that all armies engaged in the battle must be moved in the conquered countries. Then the player can move more armies if he can (more than one army remaining in the attacker country).
For the UI, we can take example on the android game. We remove movable armies from the attacker country, and consider them as reinforcement that can be dispatched between the two countries. What do you think?
We can add `movableArmies` field in the event to avoid computation in the UI in function of the game state.