Join Any Side
Join Any Side is a mod which allows you, when encountering a battle on the map, to join either side of the conflict, as opposed to only your ally, in vanilla.
When you encounter a battle, the mod will give the the option to join either side. This provides you with the opportunity to gain relations with your otherwise opponent(s). The mod will also display your relations with the leading army of each side, displayed as below.
|No Relation||Positive Relation||Aligned|