In a broad sense, it happens because of a decision to stop the japanese war machine. If you look at it jectively, that becomes the issue where there's no give. The priorities have to do with what it is the Japans are addicted to china. And if they don't stop, they're going to get the means to that off - things like oil.
Deep themes run through this show, with allegations of Japanese war crimes and atrocities in China at the start leading to eerily familiar, almost modern questions over how the world should respond. And then Dec 7, 1941 arrives...