WHAT: Legally sold U.S. military heavy weapons such as rocket launchers, grenades and automatic M16s are the basic firepower of the out-of-control Mexican drug cartels, according to leaked federal documents.

WHY: Tens of thousands of U.S. firearms and explosives are sold legally to the Mexican government each year — nearly $177 million worth in 2009 alone. Many of these weapons then disappear and are funneled to the cartels.

HOW: Those “lost” U.S. weapons are currently being shipped as military support or were stockpiled for U.S. aid programs in nations including Honduras, Guatemala and Nicaragua. Unlimited cartel cash is a near-irresistible temptation.