An Axis-aligned Brazil would force an Allied Atlantic campaign

What would happen if Brazil joined the Axis with a larger navy?

Its bases and fleet would disrupt South Atlantic planning, but overwhelming American proximity and British contingency planning would make the alignment hard to sustain.

Brazil controls ports and airfields near the narrowest part of the South Atlantic, giving any hostile fleet strategic leverage over Allied routes. A larger Brazilian navy would compel Britain and the United States to divert ships and plan operations against those bases. Geography also works against Brazil because the United States can apply economic, naval, and eventually military pressure much more readily than Germany or Italy can provide support. The likely result is an early Allied campaign to isolate or reverse Brazilian participation rather than a durable independent Axis theater.