True, but the US didn’t declare war on Germany and Italy until after they first declared war on us (much to FDR’s relief!). If they had not done so, FDR would have had a situation on his hands. Especially since he’d been angling to get into the European war for some thime.
For instance, prior to the declaration of war, we had been supplying the UK, France, etc., with arms and supplies, and even escorting the ships halfway across the Atlantic. On April 10, 1941 the USS Niblack (DD-424) depth charged a German submarine off of Iceland. U-boats also sank the USS Reuben James and the These were all acts of war by an ostensibly ‘neutral’ power. And on September 11, 1941, FDR essentially declared war on the German navy with his “shoot on sight” order.
Hitler declared war on the US in the hopes the Japanese would help him out against the Soviets.
The Japs weren’t stupid. They had their hands full without having to deal with the Russians.