http://features.blogs.fortune.cnn.com/2012/06/27/fast-and-furious-truth/Okay, so the myth states that 1) the ATF was able to arrest and charge dozens of people, who illegally purchased weapons for the Mexican drug war but 2) chose not to ('let the guns walk') in order to prosecute higher-ranking mob bosses. This is not correct.
Because of decades of NRA manipulation of state legislation, it is almost impossible to prosecute someone for purchasing a firearm in Arizona. It's also almost impossible to prosecute someone for purchasing a truckload of firearms, regardless of the clear intent behind the purchase. The ATF could not prosecute anyone, because of these extremely stringent laws. So the guns walked.
Once they had found their way into Mexico, drug gangs used the NRA-provided firearms to shoot people, one of whom was an American border guard. Then the NRA blamed Obama for their guns being used to kill Americans.