Just as with the shattered retreat mechanic we took inspiration from Europa Universalis 4 in our decision to add Coalitions. Our coalitions however are based on an Infamy value instead of Aggressive Expansion. You might recognize the name Infamy from our old games, but even though it shares the name it will work quite differently.
Infamy is limited to be within the range of 0 to 100% and will slowly decay over time based on how strong your max military potential is. When you hit 25% infamy, coalitions will be unlocked and AIs will start joining them based on how threatened they feel.Your infamy will serve as a hint on how aggressive and dangerous other rulers think your realm is. You gain infamy primarily by conquering land through war or by inheriting a fair maidens huge tracts of land.
The amount of Infamy you gain is based on the action you do, how much land you take and how large your realm already is. So for instance the Kaiser of the HRE declaring a war for Flanders and taking it is going to make the neighbours more worried than if Pomerania manages to take Mecklenburg.
Coalitions themselves are mostly defensive in Crusader Kings, if any member gets attacked by the target of the coalition they will automatically be called into the war. If a member starts a war against the target they only get a normal call to arms which they can choose to decline.
For an AI to join a coalition they will consider the relative strength between the target and themselves, how threatened they think they are and how much infamy the target has accrued. You can view the current coalition someone has against them by the diplomacy field on the character screen.
But it might not be the easiest way to view it so we also added a mapmode to more easily visualize Coalitions. A nation which turns up white is the nation you have currently selected, blue will be targetable for coalitions, yellow means they have a coalition against them and Red means they are members of the coalition against the currently selected one.