Right now if you have your kingdom/empire established you can't declare war or make peace with anyone because the button is greyed out but I have found a way to declare war and make peace with whomever you want.
Guide to Enabled Declare War and Make Peace
How to Do It
- First of all you have to download and install the developer console mod from here.
Then open the game and don't forget to tick the mod in the launcher and load into your save.
After you load into your save open the console (press CTRL and ~ tilde) and type:
- config.cheat_mode 1
This allows us to input the commands that actually let us declare war or make peace, ok so after you've done all the things I listed you need to input these commands.
To declare war type this: campaign.declare_war Faction1 Faction2 for example:
- campaign.declare_war EmpireofCalradia Sturgia
To make peace type this: campaign.declare_peace Faction1 Faction2 for example:
- campaign.declare_peace WesternEmpire Aserai
You can obviously change which countries you want to declare war or make peace.