Here is a short history of Mexico’s drug war, as told to a joint session of the U.S. Congress by President Felipe Calderon on May 20.

In 2004, a U.S. ban on the sale of assault weapons to civilians was lifted. High-powered firearms started flowing south across the 2,000-mile border. Violence increased. “One day criminals in Mexico, having gained access to these weapons, decided to challenge the authorities in my country,” he said.

Calderon did not say what happened on that “one day,” by implication the day the president had no choice but to fight back.

There is another version of history, which goes as follows: Calderon won elections in 2006 with a margin so thin (0.58 percent) that it prompted cries of fraud, persuaded his left-wing opponent Andres Manuel Lopez Obrador to declare himself the real winner, and gave Mexico the unusual and embarrassing spectacle, for weeks on end, of two men claiming they were the legitimate president.

So, ten days after eventually being sworn in, Calderon announced that he had ordered the army into his home state of Michoacan to make war on Mexico’s drug cartels.