I recently read this article by Sarah Hill in The Boston Review (http://bostonreview.net/BR35.4/hill.php) on the Mexican drug wars and how Juarez became the murder capital of the world. Two instances of the law of unintended consequences stood out:

• When the drug lord Pablo Escobar was killed in 1993, it greatly weakened the Medellin cartel. Around this same time, the US had aggressively started pursuing the Miami drug trade, which was sourced via Colombia. The result of the collective weakening of the Colombian cartels was the dramatic strengthening of the Mexican drug trade, and, in turn, the complex drug war that is going on today in Mexico.
• The US lifted its ban on assault weapons in 1994 because of lobbying by US groups in favor of broader gun rights. But it became easy for Mexican drug gangs to get ahold of them since there are now 6,600 gun stores in the four states bordering Mexico. In 2009, 11,000 guns were turned over by Mexican security to the US Bureau of ATF, and 90% of those were bought in the US.