What caused the united states to suspect the germans were planning an invasion of latin america?

Historically speaking, bilateral relations between the United States and the various countries of Latin America have been multifaceted and complex, at times defined by strong regional cooperation and at others filled with economic and political tension and rivalry. Although relations between the U.S. government and most of Latin America were limited prior to the late 1800s, for most of the past century, the U.S. has unofficially regarded parts of Latin America as within its sphere of influence, and for much of the Cold War (1947-1989), actively vied with the Soviet Union for influence in the Western Hemisphere.

Militarism is the answer.