So, sometimes Nations go to war. This is how you can resolve that stuff.
When a Nation attempts to Expand into the Territory of another Nation, the active player chooses one other player to represent the Defending Nation. Each Nation rolls a d10. If the Attacker wins the roll, they take the Tile. If the Defender wins the roll or if there is a tie, the Attacker does not take the Tile.
There are several factors that influence Conflict rolls:
The first is Territory. Each Adjacent, Allied Tile to the target Tile gives a side a +1 bonus to their roll.
The second is Power. Players may choose invest Power into either side to influence the Conflict. Each Point of Power invested increases the chosen side’s roll by 1 up to a maximum number of points (between all players) equal to the current age.
Lastly, there are a few more Advancements that can be taken to make a Nation more effective in combat.
When attempting to annex a Nation’s Capital, the defender doubles its Territory bonus.
Here are a few Advancements specific to Conflict that you could possibly use:
Metallurgy – Increase Conflict rolls by 1.
Siege Warfare – Increase Conflict rolls by 1 when attempting to annex a Nation’s Capital.
[Terrain] Focus – Increase Conflict rolls by 1 when fighting over a tile of the chosen Terrain type.