What determines when one unit in an army is destroyed?

When multiple units of the same type are in an army, one unit of that type is destroyed once the incoming damage assigned to that unit type exceeds its current average hit points. The way incoming damage is distributed depends on how many units of each type are present in the army and each type’s hidden “damage area” value, which influences how likely it is to be targeted during combat. In general, Infantry units have a smaller damage area than machine units, and thus draw less of the incoming damage to themselves.