I'll take one Texas rancher on his own land against a whole platoon of Mex soldiers, and give odds.
We just need to start shooting them when they're on our side of the border, period. We'd be perfectly justified, especially if they are armed.
If a rancher started shooting at the Mexican Military on our side, and someone got killed, I am sure the people in Washington will figure a way to cover it up, they won't want to upset Fox or Mexican's in this country. They probably arrest the rancher and give the farm away.