The development of ultranationalist and militarist attitudes in Japan's political landscape is certainly a question which continues to fascinate and occupy historians. Whilst more can be said about any number of specific events and developments with respect to these attitudes, a good starting place to understand the country's descent into the "dark valley" as it is sometimes called might be this multi-part writeup on the interwar socio-political changes in Japan.

For further discussions on how Japan viewed itself as the "guiding hand" in Asia, and how that factored into its rhetoric for war, see this post with contributions from myself and u/Lubyak. Lubyak has also written in great depth and clarity on this period, so perusing their comment history on AH is bound to turn up some relevant reading as well.