Maybe this is mentioned in the rules, but I don't notice it: say I order an Assault (units from adjacent areas converge on one area to attack). If there are enemy units in one of those adjacent areas, can the friendly units in that area join the attack? Or does the presence of enemy units somehow inhibit the ability to attack?

BrooklynMike said:

There is nothing in the rules preventing units in contested areas from executing orders. You can attack from a contested area and the enemy units in the contested area would not participate in the battle. The combat rules only mention enemy units in the target area, so enemy units in origin areas are ignored.