The US Army on the Mexican Border

[Ken White]

And then there was the case of Ezequiel Hernandez.

http://en.wikipedia.org/wiki/Ezequiel_Hernandez
Esequiel Hernández Jr was an 18-year-old U.S. citizen and high school student killed on May 20, 1997 by Marines on the United States–Mexico border in Redford, Texas. He was herding some goats, but the Marines thought he was some kind of smuggler.

I'd be curious how this incident changed (if any) the sort of rules of engagement the U.S. military operates under down on the border.

over the strong objections of DoD and some in Congress. Most of the objections were to prevent incidents like the one you cite; that and the bending the rules on Posse Comitatus.

The Hernandez incident is sterling example of what happens when the government tries to garner political and propaganda points instead of doing things right. DoD and the service got vindicated but that didn't help Hernandez. You did note that Hernandez fired first -- getting shot at, even with a .22 gets one's attention -- and that several investigations resulted, correctly IMO, in no punishment for the Marine. Sad and unnecessary for all concerned.

In the event, the ROE were changed not only on weapons use but on deployments in general for such purposes. Essentially, no Active Forces are routinely deployed (the ARNG is not subject to Posse Comitatus restrictions unless Federalized) and those that are operate only in a support role.