Adolph hitler declared war on the united states. The germans aren't going to get anything for it. But britain have already had talks about what they might do should a war break out and we find ourselves working together. They're able to craft an arrangement for how this war's going to be fought. And then after that's done, we will go over and grind the japanese to powder at our leisure. That would be extremely unlike him to ever make a bad deal.
Japan's rising sun goes supernova and engulfs a huge area of Asia and the Pacific. A war without mercy begins to develop infusing the whole conflict with a savage vibe.