This doesn't even seem that far fetched at this point. The economic influence of the USA is being eroded at every turn. Their military capabilities could very well turn out to be their last hope one day. South America stands virtually no chance against even a decadent USA. It's actually embarrassing how weak South America is.
The problem is that the main military enemy of South America is Other Bits Of South America, especially internal enemies. That's why Costa Rica has no military: can't have a military coup without a military.
Easy one, then. The administration will kidnap the president of Bolivia, install a replacement and strike a minerals deal.