John Perkins' book "Confessions of an Economic Hit Man" explains American foreign policy better than any of the academic tomes you might read on the subject.
In a nutshell, the game is played this way: People like Perkins work for consulting firms, and their job is to entice a foreign head of state to go deeply in debt. They do this by greatly exaggerating the economic returns on big projects such as dams and electrification systems.
The payoff comes in two ways. The foreign country hires American contractors to build the systems, and they make big profits. Then, mired in debt, the head of state will do what the United States government tells him to do. If he proves too independent or too honest to accept bribes, then he will be removed from power, either in a coup or in an accident.