My case is very interesting in the assumption that the enemy army arrived in California in the mother mountain range they will surely die or, they will surely can choose to go to U.S. or MEXICO, in any case they are 3 army bases per foe with that Mexican army less people so the time of a regular large scale battle is likely to be over in 1 week or more, with the combined forces of the 2 countries because who cares the American army is going to take some action.
In the case that they arrived in Mexico City I wonder that they will be checkmate with the Mexican army entering the zone because they have a parade of soldiers every years, so things will be turning very funny because the Mexican can choose the path of the friendly foe.
(If they success in the negotiation the Mexican will take everything of the gate zone and not kill them with guess what? Soda and tacos and even they will be die of diabetes,)
Or make business in addition to the U.S and problem solved for the two in any economical way.