Historically, they kinda' do have a pretty good excuse to be grumpy about it, after a fashion. Technically, afaik, they were turned into an expansionist empire by primarily US interests.
In the mid-1800s japan was forced to open trade with foreign nations, by a mostly american fleet of ironclads, kickstarting their turning from a tradionalist, extremely insular society, with few interests in matters outside of Japan (and even much of that came from europeans forcing their way in, after a fashion).
At the time of WWII they'd been building up steam, for nigh on a century, towards a confrontation with all the nations who'd been using them as a doormat at one point or another, with a fair bit of people reckoning that was the way to end that sort of thing, I imagine.