The classical argument in favor of marijuana legalization rests on personal liberty. Why, proponents ask, should the federal government tell free citizens what they may consume? It is also one reason why many conservatives fear it. They worry that legalization will mean more pot heads, an increase in the consumption of hard drugs, and a decrease in the quality of life for the sober and for society at large.

Former Brazilian President Fernando Henrique Cardoso believes the opposite would occur. In an interview here last week he told me that his embrace of global marijuana decriminalization is aimed at reducing all drug use, bringing down violence, and diminishing what he sees as a serious and growing threat to democracy in Latin America.

Out of politics since 2003, when he finished his second four-year term as one of Brazil's most successful presidents, Mr. Cardoso is now a high-profile international advocate for ending the war on drugs. But he once held the opposite view.

Mr. Cardoso explains that as president he used traditional methods of "repression and prevention" to fight the drug problem. He is quick to add that neither worked. "Eradication was a failure," he says. Even though marijuana plants were destroyed—the government proudly took pictures of its handiwork—"later on, again, the crops were there." Meanwhile, the state made an "insufficient" effort toward prevention, in part because Brazil's drug problem "was not that bad at the time."