What was missed in Al's response (though perhaps he just hasn't gotten to it yet) is the rather laughable implication that George Bush Sr. chose to invade Panama because he found out that Noriega was involved in the drugs trade.

Obviously that was the excuse given, and is the received mainstream doctrine, but does anyone reading this believe that line?  It was a pretext for an invasion desired for other reasons.

Even barring Al's explanation of why an invasion of Mexico over drug policy is unlikely, the historical example of Panama itself does not succeed in establishing a precedent of invasion over drug policy, as drug policy was not the cause.

It does set a precedent that if Mexico were to move toward drug legalization, that this would provide the US Government a pretext.  In the event that the USG would have some other compelling reason to want to invade Mexico, and one they'd prefer the public not to know or debate, they could pull the pretext out of their hat.  So there's still a potential risk there for Mexico, but that's quite different than what was implied in Baylen's comment, and perhaps Al's answer applies just as well to this different scenario.