I have been told Brazil joined WW2 after, allegedly, the Nazis sunk a ship near Rio de Janeiro. As far as I know, Brazil had decided to stay neutral in the war and then-president Getúlio Vargas was actually friendly towards Hitler, so if the country did join the war, they'd have joined it on the Nazis' side. So why would Hitler attack the country? Didn't he have better things to do?
I've heard that, actually, the US were the ones who sunk the ship, they just blamed it on the Nazis so Brazil wouldn't aid the Nazis and would instead help the Allies (which's exactly what happened). This makes the most logical sense to me, but considering that's not the 'official' story, surely there is a good reason for why Hitler would give that order...
The German and Italians sunk Brazilian ships and killed most of the Brazilian citizens aboard those ships. They considered that the agreements between Brazil and US (including installing and American military base in Northeast) were a violation of the neutrality and that Getúlio Vargas had picked a side. At first, the president didn't want a war, but the Brazilian people protested for war because of the attacks against our nation, and that's why Getúlio declared war on Germany and Italy. About the rumors that the Americans had sunk the Brazilian ships and blamed the Nazis, I can tell you it's not true. The Germans put on their logbooks all what they have done in the day, and some of those logbooks show us that they really did sink the Brazilian ships.