Battles
When enemy circles overlap, the shooting starts. Battles take hours, not seconds.
Battles start by themselves when enemies meet. Every unit picks its own targets, going for what it is best against first.
Army and Navy groups in a battle cannot move until it ends: there is no retreat. Aircraft are different, and can fly away at any time.
More of your groups, and your allies, can join a battle by moving close. It ends when one side has nothing left standing, or when nobody can hit anybody any more.