The U.S. Joint Forces Command "Joint Operating Environment (JOE 2008)" report names Pakistan and Mexico as two states that could suddenly collapse (link):

"The Mexican possibility may seem less likely, but the government, its politicians, police and judicial infrastructure are all under sustained assault and press by criminal gangs and drug cartels. How that internal conflict turns out over the next several years will have a major impact on the stability of the Mexican state. Any descent by Mexico into chaos would demand an American response based on the serious implications for homeland security alone."

First, there's only the possibility that their government could collapse; it's not a certainty. But, if it did happen it would obviously cause enormous problems for the U.S. We'd probably still have legitimate trade with Mexico; if we didn't California and other states would be in financial difficulties. So, we'd still need to allow some border traffic, even if mostly just for goods. If we tried to prevent "visitors" who might decide to stay, border communities inside the U.S. would suffer greatly without that income.

At the same time, the patriotic choice would be to do everything within our means to stop the massive unauthorized flow of people northward into the U.S. that would result. Unfortunately, many of our leaders oftentimes don't make the patriotic choice; many of them would try to let in as many as possible. Some of them might welcome that as a way to push the North American Union. And, it's not outside the realm of possibility (based on things that have been done in the past) that a crisis in Mexico could be manufactured in order to lead to a NAU.

In any case, our elected officials would receive pressure to let in as many fleeing Mexicans as possible from the United Nations, the far-left, racial power groups, and those elected U.S. officials who frequently act more like paid agents of the Mexican government, such as Gil Cedillo or Bill Richardson. If we were however allowed to halt the flow, we'd need to establish a buffer zone all along the border, perhaps using non-lethal weapons as a shield. Actual battles might occur along the border; see this for a complicating issue. In addition to costing an incredibly amount of money and weakening our ability to fight wars in Iraq and Afghanistan, that would put the aforementioned groups in a tizzy.

Meanwhile, what would the millions of Mexican citizens - and U.S. citizens who think of themselves more as Mexicans than Americans - living in the U.S. do? They'd certainly put pressure on the U.S. government to let in as many of their countrymen as possible. If we were able to reduce the flow, those Mexican partisans inside the U.S. would probably not be very happy at all. Things could turn violent, and Andres Oppenheimer's "Latino Intifada" could result. Note also this 2005 warning that shutting off illegal immigration could result in Paris-style riots in Mexico, and in 2007 McCain warned that not passing comprehensive immigration reform could result in Paris-style riots inside the U.S.

If there weren't millions of Mexicans and a fair number of de facto agents of the Mexican government inside the U.S., any collapse would be much more manageable. The fact that 10% of Mexico's population lives in the U.S. would make the situation much, much more difficult.

The only safe solution is the one mentioned at the intifada link: we need to back away from the current situation slowly but surely. That means working to sharply reduce illegal immigration, blocking all amnesties, and encouraging current illegal aliens from Mexico to return home and reform their own country. Mass deportations would not work for various reasons.

The easiest and most effective way to solve this issue is to discredit politicians over immigration matters. If many of those who support massive/illegal immigration were discredited there would be much less support for illegal immigration and amnesties, and the problem would be lessened over time. If, for instance, McCain had been pressed hard on this question that I posted in June 2007, this issue would have been brought to the fore and McCain might not have been the nominee. And, he would have been much less able to push amnesty or make his various pro-amnesty comments.

The bottom line is that you need to take action. This page describes what you need to do. If you can't or won't do anything on that list, then at the least send that link to your friends, post it to forums, and encourage people to take those steps. Otherwise we could end up in a very costly and dangerous situation.