Duncan Currie writes this about Mexico's drug violence and its relation to political and economic issues in National Review Online:

Drug seizures make for good photo-ops, but they rarely have a big impact on the DTOs [Drug-Trafficking Organizations--md]. By contrast, killing or arresting senior drug lords can cripple their gangs. Over the past few years, Mexico has taken down a long list of kingpins. Alas, these victories have not yielded a sustained reduction in violence.

The only way for this to make sense is if success against drug gangs is measured in some other way than decreasing violence. And it may very well be: it is perfectly logical to measure "success against drug gangs" as decreasing the quantity of drugs flowing across the border. However:

1) I know of no evidence that any decrease in drug traffic has actually occurred. In fact, the continued violence seems to indicate that there is still a profit in running drugs across the border, so I would argue that the level has probably remained the same or increased slightly to allow the traffickers to finance their feuds.

2) I get that Mexico is a close ally of the US and that they are sort of doing our bidding by attempting to eradicate/decrease their local drug trade. But to me it seems like their more important interest is keeping violence down within their own borders. Matt Yglesias has a couple of posts that touch on this point, both directly and indirectly.

Finally, if knocking off leaders is increasing violence in Mexico, maybe they/we should try another strategy. That is the subject of one of my forthcoming papers, which I will share here as it progresses.