WASHINGTON—President Obama’s visit to Chile, Brazil and El Salvador next week should not raise hopes about a grandiose outcome. The region’s positive dynamic—the explosion of the middle class and the marginalization of the extreme left—is one on which the United States has had little bearing. The negative one, i.e., the violence related to the drug war, partly derives from a U.S. policy that will take years to change.

Latin America and the Caribbean have a long way to go: Only Chile and Barbados are ranked among the 50 most competitive economies. Its universities graduate six social scientists for every two engineers. And the money flooding Latin America today stems partly from Chinese demand for its commodities. But there has been a leap forward due to investment and trade. Poverty has dropped to one-third of the population—30 million Brazilians joined the middle class in the last eight years—and the economies of Peru, Colombia and Panama are booming, while Chile’s is back up after its earthquake.

The last time the U.S. engaged the region in a common vision was the Free Trade Area of the Americas. Once that failed, Washington lost interest. In some respects, economic growth has taken place despite U.S. policy. Six major Brazilian exports face protectionist obstacles in the U.S. market, and Congress never ratified the Colombian Free Trade Agreement signed almost five years ago. Progress is mostly owed to a widespread political consensus in favor of market democracies. As reformed former Salvadorean guerrilla Joaquin Villalobos has said, “Natural resources, external aid, free trade agreements and loans don’t have the effect of political maturity.”

A byproduct of this consensus has been the waning influence of the leftists of Cuba and Venezuela, supported by Bolivia, Ecuador and Nicaragua. One of Obama’s hosts, El Salvador’s Mauricio Funes, has trumped efforts by his party, the Farabundo Marti National Liberation Front, to allign the country with Venezuela, while Uruguay’s Jose Mujica, a former guerrilla, is now a grandfatherly social-democrat. Even Paraguay’s Fernando Lugo, who once flirted with the revolutionary coven, is now luring foreign capital.

Hugo Chavez is struggling with a chronic debacle. His main South American ally, Bolivia’s Evo Morales, is now loathed by two-thirds of his country, according to opinion polls. Nicaragua’s Daniel Ortega is trying to get himself re-elected despite a constitutional ban, but that is partly a courtesy of the divided opposition.

Again, the shrinking of the loony left—ultimately more transcendent for the future of Latin America than the economic takeoff—owes little to U.S. influence.

The other side of the coin is the law-and-order drama fueled by the drug war. Here U.S. policy is a factor. Under pressure from Washington, coca cultivation shifted from Peru and Bolivia to Colombia in the 1990s. When Colombia turned the screws, it shifted back to Peru, where the area under cultivation has inceased by 70 percent and production has trippled. The same game of musical chairs is apparent with regard to trade routes. When the U.S. shut off the Caribbean corridor, Mexico replaced it. After thousands of people were killed in Mexico, some cartels—surprise, surprise—moved south, wreaking havoc in Guatemala, where the rate of violence is four times that of Mexico, and Honduras, where the rate is even worse.

Even though several American states allow medical marijuana and personal consumption is no longer heavily persecuted in the U.S., Washington’s drug policy in Latin America has not changed one bit. Calls by a wide spectrum of Latin American political and intellectual figures as well as major institutions to ditch the repressive approach have fallen on deaf ears. Three former presidents—Brazil’s Fernando Henrique Cardoso, Mexico’s Ernesto Zedillo and Colombia’s Cesar Gaviria—issued a thoughtful appeal in 2009 to no avail.

The result is a nightmare taking place in countries where the flow of arms coming from the U.S. is endless. (Mexico has confiscated more than 100,000 automatic weapons over the last five years that came from across the border.) Demand for drugs in the United States, meanwhile, has remained stable and prices have dropped because of the unstoppable supply.

Obama probably knows it but does not have the stomach, at this point, for the protracted fight that a change in drug policy would entail. And until it happens, as Mexico’s Felipe Calderon recently told me, it is unrealistic to expect any serious Latin American country to defy Washington on its own.