Unit attacks and only attack the first target that approach into its aquistion range or has attacked it. Other units won't be attacked as long as the first attacked unit is alive.
Otherwise, neutral units somehow "bug" if they has passive ability "Poison" and they will attack everything until all units are poisoned...
You can create an system for the threat itself, no otherway to do it (I think)
And, theres not trigger function for "threat".