Why would they ally themselves with Germany and get involved in a European war?
Sure, you could say the European powers were distracted, and an axis Japan could probably get in quite some early victories; but the US isn't going to stand for it.
No matter how isolationist the US is, there is simply no way the US is going to let Japan run rampant all over the pacific. No to mention the fact that US-Japanese relations were finally improving after the Japanese withdrawal from China.
Besides, wouldn't this put Japan at war with the USSR?