From a report by the U.S. Joint Forces Command (pdf):
In terms of worst-case scenarios for the Joint Force and indeed the world, two large and important states bear consideration for a rapid and sudden collapse: Pakistan and Mexico. . . .
The Mexican possibility may seem less likely, but the government, its politicians, police, and judicial infrastructure are all under sustained assault and pressure 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.
And this from a report by Gen. Barry McCaffrey, former drug czar (pdf): “Mexico is on the edge of the abyss—it could become a narco-state in the coming decade.”