Nothing much will be done until there is an event like the attack on Columbus, NM, by Poncho Villa. This is becoming more likely, because of the falling numbers of economic illegals to act as cover for the armed and murderous drug smugglers and professional killers.
Likely, a drug convoy escort of Zetas dressed as Mexican military, or Mexican military themselves, armed with military heavy machine guns mounted on their vehicles, will meet up with a group of Americans. Border Patrol would flee the more heavily armed Mexicans, and wisely, as they have done before, but this group will not flee, not understanding the threat.
The Zetas will probably slaughter the majority, but at least some will get away. Then Washington will do everything in its power to downplay the incident, insist that they are working with the Mexican government, etc., etc. But the event will be too bloody for them to ignore. They will try anyway.
But from that point, US private militia forces will have had enough, and they will set up an ambush for the next incursion. And they will also video record the firefight, and possibly a dozen or more dead Zetas. This will finally force the issue.
Undoubtedly, the US government will arrest the militiamen for murder, which despite their best efforts, will not be a closed trial with no media coverage. The USG effort will still be lame, sending some National Guard soldiers to patrol an unimportant and unused section of border.
But from that point on, any armed Mexican who crosses the border will likely be fired on without warning. Unarmed Mexicans may get warning shots.
Walls and fences work wonders....if not they need to take the one down around Ft. Know and scrap the metal; goobermint needs some cash.