Could you imagine the actual amount of ass kicking Mexico would receive of they declared war on the US?
Well, probably not. The US might just fortify its border with troops and material and try to talk them down. All while blasting anything that comes across the border.
America went to war with Mexico when Mexico tried to enforce its border at the Nueces River which is north of the Rio grande. The US recognized the Rio grande (the current border). It started the Mexican-American war
Border incidents are always tense. Especially when the governor of said state, along with current presidential candidates, are calling for militarization of the border to protect from "invasion".
Surely you see the difference there though. An official position versus an individual with no authority committing a crime. It's only an act of war to the most sensationalist.