I was idly looking at the Rainbow War Plans when War Plan Purple caught my eye, which provides for a conflict between the United States and any given South American country.
This got me to thinking as to how the United States might end up at war with a South American country in the 1930s. What POD would we need? What kind of crisis would push the US into war with, say, Chile or Brazil? Given the isolationism of the era, it'd have to be worth our while or in retaliation for something crazy.
On a second point, how would such a war be conducted? I imagine the Navy and Marines would do much of the heavy lifting in such a scenario. Might this lead to an earlier building of aircraft carrier theory?