You're thinking in a normal under rule of law situation. I'm thinking of without rule of law. And you forget that the same countries who support Haiti militarily support the Dominican republic as well. All with the exception of Israel, are currently disarming their military's to maintain operations in Ukraine. Even in a rule of law situation, I doubt they'd intercede outside of helping evacuate people. Haiti's military, with its grand total of 2k people, will not leave their homes in response to mobilization orders to attempt to restrain or intimidate 5 million fighting age adults who will be vastly better armed and organized than them. Mercenary army's will not involve themselves in that either, because the US has in the past gone after anyone they thought might turn a banana republic into a rouge state on their doorstep. The Dominican military has a little under 60k and in theory they might be able to make some moves to slow the Haitians down, but they'd ultimately be futile. I'm expecting that foreigners, the rich, and the Dominican national government would flee, than fight and leave everyone behind to fend for themselves.