In 1970, supposedly the cia executed an operation to overthrow the president of Chile. What were the motives?

If this military coup occur under the guise of the president asination, and being reminded that this is during the vietnam war, what did The United States have to gain? Today, with the war in iraq and escalating tensions with iran, insiders are saying that there are preparations in progress to execute a similar operation in Venezuela as was done in Chile? Again, what is there to gain by a military coup in Venezuela?