Look at The ABC Conferences.
We seized French ships and were very worried about a Nazi attack in South America.
The details of American military thought in the summer before Pearl Harbor are amazing.
The ABC conferences did not assign the British responsibility for the Pacific. See my prior post.
The seizure of the French ships was due to our various Neutrality Acts as we had guaranteed the neutrality/ownership of the islands the crews of the French ships seized/liberated.