A firefight takes place if there is an enemy detachment within 15 cm after a player has resolved all of the close combats that they triggered in this Assault phase.
Note that it is possible for a detachment to be involved in a close combat and then take part in a firefight immediately afterwards.
If a detachment has any units within 15 cm of an enemy detachment at this stage then you must resolve a firefight between the two detachments. When several firefights occur in the same turn, the player with the Initiative may decide the order in which to resolve them. [TODO: Consider whether instead the first player should decide the order while they resolve the assaults that they trigger, and then the second player should get to do the same.]