they just attacked us, rather like the terrorists. The orders to the Japanese Embassy in DC were delayed in decryption, so they weren't delivered until well after the attack at Pearl Harbor. Germany did indeed declare war on the US a few days later, a move regarded as one of Hitler's dumber mistakes. Had he not declared war on the US, we might've had a hard time convincing the public to follow the "Germany First" strategy following the Japanese attack. American and British strategists had agreed on the "Germany First" plan long before Pearl Harbor.

Now, both Germany and Japan did have largely heterogenous populations. The Germans achieved this through concentration camps and other unpleasantness. The Japanese have historically been very homogeneous (other than their native Ainu population, similar to the aborigines in Australia) through restrictive immigration and long standing racism.