First I thank you to take the time reading my post. It's a subject that count a lot for me, as raising awareness about it, I also take the opportunity of the #discuss tag lunching to begin one about it. Then I want to say when I talk about USA, I talk about his government, not people living in. And finaly, my english isn't perfect (far away from it, I'm a French native speaker) so I maybe made mistakes, but hope you will understand all.

Even if isn'te the part which I will talk, we have to start by the begining : Spanish colonisation of South America, during the 15th century, strongly marked the continent history. Peoples eradications, plundering of resources, appropriation of territories... This part is well known by each one, learned at school (in France) under the "Discover of the New World" by Christophe Colomb, as if all that was on the continent before didn't count before Spanish Catholics arrival, and like if it was a chance for people there to evolve.

However, it counted already number of rich civilisations (as culturally, than spiritualy, and even as economically, with the amount of minerals such as gold and silver they had collected), but we are getting away from the subject : Why South American Countries are still poor our days ?

During the 19th century, some countries were taking their independence, while the last colonies were passing under USA control, winners of the Hispano-American war of 1898. This event could have been the begining of their development, as North ones had developped themselves before. But the objective of the victors wasn't to free them from Spanish scavengers unfortunatly (Themselves [USA] already were exploiting resources of the island under their control since 40 years...) but in reality to take control themselves on the resources and territories (Like did France on Africa's continent and many others countries for exemple).

In the 60s, number of South American countries having taken their independence, USA started their political and economical annexation, under the guise of helping them progressing : The IntraAmerican's conferences in Punta Del Este (Uruguay) had apparently the objective to grown annual income by resident, stop illeteracy of the adults, stabilize the prices reducing inflation and deflation, establish democratics governments (insidiously, this objective was for gang up south america against the 'communist threat' after Fidel Castro takeover, for not loosing the capitalist potential), equitable income distribution and, of course, reform their economical, social and agragian systems to conform them to american model.

Thus, USA prposed an economical help to the countries signing the contract, in return for social, agrarian and economical systems reformed at their image. They win thus the grasp on all the resources and clients to sell back to them to products they make with (much more expensive), depriving them from their industries and way to progress. A part of this, they continue to make them credits with interest (which will contribute to sink them during the USA crisis, that had make grow the dolar price and so the debt of this countries in the same time).

The amount of the economic help given was calculated by two searchers to 22,3 billions, which we deduct the resources and purchases transfers :
All the profit that this countries made returned to USA, for an amount often higher than the new investisments
Moreover, USA obtain that all the purchases, of industrial equipment and vehicules, made with the money lent, go back to them or allied firms. A studiy made in 1967 by A.I.D show that 90% of all the purchases were down to american firms
Through an article of ensuring that this amounts was invested to them...

The treaty was approved through a strong lobbying campaign, and many political actions against the countries which didn't wanted to sign : help to the takeover in Brazil, one the ultimate which didn't want to sign. Deployment of police and army forces in Dominican Republic, plan LAZO in Colombia, and economical pressures in Argentina, Ecuador, Honduras, Peru and Guatemala. The famous disembarkation in the Pigs Bay (Cuba) also happened the same year.

In the end, USA, probably with assistance of others occidental multinational firms and banks, ensured themselves the fourniture of raw material for free with the circuit to sell it transformed more expensive to the same countries that was giving them it for cheap. They also put their ideology in those countries, with all the disastrous consequences we know, puting their model inside their education (child and adult formatage), social (overconsumption, materialism...), agrarian (machinery, GMO, chemicals products, hormones injections...) making it pass from a familial and peasantry system to agro-industry. It with all the consequences on economy, health and philosophic way (from ancient people valors, we can now observe there rich ones with pick-ups, bigs cars and wanting to be as rich as occidentals to can consumm things that don't really make them happy, and most of poor ones hoping the same). Like this, USA sinked those countries for his own interest.

Sources (Except the book, all are in French, sorry) :