Sequence of Play:
1. Place New Units
2. Intrigue Phase
3. Play Card...This Phase is when an you could play an Event Card that could place Enemy of Rome (green) cubes in an area controlled by your Legion Cubes.
So not a reinforcement action, but planning for the card play by using the Phase 1. unit placement to add Legion units to the area.
True, but still there would have to be a combat to resolve the situation where there are 2 factions in the same area before the start of the next turn. Maybe I'm not following your line of reason as it applies to the original question.