It's possible that Iran, or al-Qaeda, or Russia, any of them, are the biggest threat to the US in the next 8 years. That, my friends, is a luxury. That our greatest threats are all half a world away, that the worst consequence of defense failures for our civilian population was as single attack in an almost-70-year period is spectacular. We can spend all of our time worrying about whether Iran is funding a militia group with an effective operating radius of 100 miles.
That is, of course, as long as these 3 countries are our biggest threats. They might not be. I allude so cleverly, of course, to Mexico.
"Mexico?" you ask. "I vacationed there once. You can only drink bottled water, but how's that so bad?" So thought I once, as well.
I also maintained this opinion of Mexico as a rather poor but otherwise relatively inoffensive place over the past 8 years despite A) about half of the US becoming fixated on keeping Mexicans out of Texas, and B) increasingly frequent reports on America's Most Wanted (which, I admit, I watch at home during Thanksgiving and Christmas breaks) about crazy Mexican drug cartels abducting/killing Texans.
But the US Government has caught on. The Department of Defense is starting to seriously worry that the drug cartels could take down the Mexican Government.
Is it time to be terrified? Probably not. But the cartels have become extremely powerful, and momentum is on their side. For many local police, they must either accept bribes or be killed at night--"silver or lead," as it's called. Many police are still trying to crack down, and big fights are happening--generally between drug gangers and cops from calmer parts of the country. But the corruption runs pretty high, and at some point, impunity starts to take over: officials can stop pretending that they're part of the forces of order. It hasn't happened yet, but it might.
When that happens, Mexico is already a failed state. But other similarly catastrophic events could happen in this drug war: if Mexican leadership is assassinated, the rough and hectic transition period could be so ineffectual that the government would "lose its hold" of the country (which I have found in my studies to be a very elusive concept to operationalize in a country: the place to start looking is the Fund For Peace's Failed States Index). The drug war could get so bloody and miserable that people start to displace to avoid it--and the economy crashes. Any of these things could occur.
So what would happen if the Mexican state failed? For the US, a few pretty bad things. First and most obviously, there would be a huge rush of refugees to Texas, New Mexico, Arizona, and California. Oh, and given a pesky treaty on refugees that the US signed back in the 1950's, it would be obligated under international human rights law to take all these refugees in (particularly if the UN declares a certain level of crisis in the country to be present, which happens in the General Assembly, leaving the US veto useless). The US would either have to snub international human rights law in a very big way (something that would be very costly for the US' reputation, as well as an act that would undermine the world order that the US worked so hard to create for the past century), or take them in and deal. And then, the US would have to choose between shouldering this obligation itself (which would be financially ~impossible given current government debt) or swallow its pride and knock its status in the world by accepting large humanitarian aid from Western Europe (not that the Western Europeans would care much beyond jockeying and cultural competitiveness, but the Russians and Iranians would likely become quite bolstered).
The other problem, of course, would be law enforcement. The US would have to turn its southern border into a war zone, and send huge numbers of (unavailable) troops to provide law enforcement in the region and try desperately to keep the border watched and safe through the rush of refugees.
Can this be prevented? Almost certainly, but it will take some smarts. It's something Mr. Obama will have to put on his plate, right next to Gaza, the financial crisis, Iran, al-Qaeda, Afghanistan/Pakistan, and all the other plans he hopes to put through. US demand is a big contributor to this drug drade. Curtailing demand would help, but the only serious way to do that would be to legalize the production of drugs inside the US, reducing the need for foreign supplies (I say this due to overwhelming evidence that the War on Drugs isn't really going anywhere). Another way to do it would be to build a Berlin-Wall-esque station at the border to make sure the drug supply and demand are permanently separated, despite the "pressure" for goods to flow (think of trying to keep a vacuum separated from a high-pressure vessel with a membrane between the two). It seems unlikely that either solution is going to look attractive.
It's a question for which I don't think there is a clear answer. Supporting the Mexcian government with US aid/intelligence/training is likely to help some. It is certainly the cheap and low-effort solution, but it's unclear as to whether it would be enough.