The United States has a lot of money and an endless appetite for opiates, narcotics, and weapons of all kinds. The varied environments of Latin America include perfect growing conditions for plant-based drugs, and economic conditions that often favor illicit employment and corruptible public officials. (Not that underemployment or corruption are unknown north of the Rio Grande.) Given its central position between suppliers and customers, northern Mexico has become the focus of increasingly desperate -- and ineffective -- efforts to interdict drugs.
Jonathan Rivalt and Richard Johnson of Ontario's National Post have researched the geography of both the trade and the violence, mapping the connections that bring drugs into Mexico and the connections between the cartels and market areas in the United States. (As many online commentators have noticed, the research inexplicably stops at the US-Canada border, but the maps are instructive, nonetheless.)
As I noted recently, President Calderon has paid a political price for his extreme cooperation in the US-led "war on drugs," but it is the people and communities of northern Mexico that have paid by far the greatest price. The graphic below can be more easily viewed in the original National Post article.