Have South America enter European alliance system. For Example Brazil is CP, Argentina is Entente. If you still have WW1, you will get major USA action down there.
This gave me an idea: I believe that if, for example, the US had joined the CP, you might have a pro-American Brazil against a pro-British Argentina, for example.
After all, the main influential powers in South America during the early years of the XX century where the US and Britian, so, if they fight between each other, that fighting might extend to their allies...
Think of a greater chaco war, for example, between one side supported by the US and the other by Great Britain. At first the fighting is done only by South America, and the US and GB only give indirect support to their side. But then the conflict escalates and at least one side (let's say, the US) starts sending troops.
It's that or you've got to have one of US' rivals get much more influence in South America (Germany?, the USSR? Japan?). The main reason why the US hasn't fought a major war here is that the region has been mostly in the American sphere of influence or, at most, during the early XX century, under the British influence (being GB a British ally). So you'd need either Britain to be hostile to the US when it was still influential in the region, or have other outside power be influential enough to make the US see this as a threat serious enought to justify intervention. In this sense, I think the cold war, the fear os fsoviet inflience and a Communist revolution in one of the great South American countries might justify this...