[Case Closed]

Japan was not really planning on starting a war with the US. Despite our declared neutrality we put some heavy duty economic sanctions on Japan which screwed over their war effort. We stopped giving them oil and we started to back the Chinese.

If we had remained neutral and allowed Japan to massacre the Chinese people with biological weapons? I don't know what would have happened, I assume Japan would have a strong hold on some terroritory in China today. It's hard to say though since so much of history would have been drastically changed had they never done the whole pearl harbor thing.

Although with the sanctions their war effort was slowing down so they probably would have not gone much further after that.