Conflict and Explore
Each of your mobile units operate under one of the two missions: Conflict or Explore. You can have all of your units obey the same mission, all of your newly built units obey one order while existing units obey another, or various units can be set to follow one or the other of these missions.
At the start of each campaign, all realms begin with the Explore mission for all their units. Under this mission, your units move around the map as you tell them, but they retreat one hexagon if they enter a hexagon that contains a unit belonging to a non-allied realm. The Explore mission means that your units don't automatically attack any non-allies they meet. This gives you a chance to decide if you want to attack or try to talk to the realm encountered.
If you want to take a more aggressive (hostile, you might say) approach to encounters, choose the Conflict mission. Units under this mission do not retreat when they meet non-allied units. They stay in the hexagon with the other units and attack. Of course, if the other units are under an Explore mission, they may move out before you can attack. While this order doesn't win you any friends, it may enable you to gain territory more quickly, since you never voluntarily retreat from encounters.
One important point to mention is that all units that follow a lead unit are assumed to have the mission of the lead unit. In other words, if 20 units with Explore missions are following one unit with Conflict mission, when that group meets a unit of a non-allied realm, they operate under the Conflict mission. Once a unit's Follow order is canceled, it operates under the mission it was originally given.