It really does matter what the intentions of the "democratic" government are. Democracies can be expansionist, isolationist, racist, hawkish, dovish...
Based on Japan's culture, they'd probably be fairly militant, and it would be a limited democracy with a fair dollop of authoritarianism; perhaps a VERY strong chief executive (who would 'speak for the Emperor' for PR purposes) and a weak legislature.
There's no reason such a government couldn't behave almost exactly like OTL's Japanese government, right up until 1945 or so. If they don't have to stand for regular elections, there might not be any noticeable difference in the war.
Or, of course, a wave of populism could sweep Japan, and a very 'western-modern' government could be put in place. Such a government might even side with the Allies, leading to shorter WWII.
Or just about anywhere in between.