Then they just disembark the buses, and continue on foot.
If the caravan army continues on foot, they soon run out of food and supplies. They are forced to turn around and go back.
Northern supplies are cut off. They will march where the food is located.
Bomb a few key roads per day. The longer Mexico waits, the fewer Northern passable roads.
It is simple, and the best solution. Nobody gets hurt.
But bomb early.