GK, If you are implying that the other regional powers should have forgotten Japan's imperial past that I suggest a quick International Relations primer might be in order. State's memories and policies go back hundreds of years...1945 is not even history in that region.
But aside from mere memories, analysts know that Japanese imperialism existed for a reason - and those reasons are as valid today as they were in the past - small country, over populated, cramped, lack of resources...all the usual stuff.
Japan is a country in trouble and without the US as a massive strategic partner, potentially in bigger trouble still.
So a country with a recent violent imperialist past, big internal problems and a consistently vast (and disguised) military spend is talking about increasing its already huge (in both absolute and relative terms) military budget, increasing the role of its armed forces (sorry, self defense forces) and changing it's constitution to allow a more "interventionist role and you think that shouldn't make China and North Korea nervous.
I'd suggest it scares the crap out of them.