My history teacher in high school said the official excuse to join the Allies was the sinking merchant boat done by a German submarine. He also said there was

My history teacher in high school said the official excuse to join the Allies was the sinking merchant boat done by a German submarine. He also said there was no direct proof it was actually the Germans, and it may as well have been the US to force us to join on their side. Which ultimately led me to find a pro-military website contesting this whole idea, and the author presented convincing arguments like, Brazil was sending resources to the US, so why the hell would they sink a ship that would bring resources to them? Sadly, I cannot find the link. And it turns out the widely accepted theory is that Germans actually sunk the merchant boat, and it wasn't a covert US operation.