“IIRC, they have shot across the border before, but not crossed over by air. Am I wrong on this?”
My SILs Step-father used to work the border between Tucson and East a few miles and he and his crew were shot at regularly but not allowed to return fire. To his knowledge, no USA personnel were ever hit.
I know a Marine reservist who worked on the border wall in Douglas, AZ. Same experience.
We’re just too darned nice for our own good, sometimes.
Where are our anti-aircraft weapons?