A co-operative campaign allows two players to play a full campaign against AI opponents. It is largely similar to a standard campaign with the addition of a shared victory condition: both co-op partners must retain possession of their capital, but required provincial conquests are shared between them. Diplomatic states are also shared between partners, meaning that if one declares war against an AI faction, their partner will also be at war with that same faction. Similarly, ceasefires and alliances will also be shared. Within a partnership, the field of view on the campaign map will also be shared, including line of sight from units and buildings, plus unveiling hidden sections of the campaign map.
During a co-op campaign, it is possible for players to share troops during their battles. This process works both ways so that partners can swap troops back and forth between them as often as they like during a battle. Even if one player has no troops involved, he can still enter the battle and be ready to receive shared units whenever his partner is ready. Once the battle is over, shared units return to the control of the partner who recruited them.