There is a lively scholarly literature, with important implications for international relations, on the causes of imperialism and imperialist expansion. Those who routinely offer simplistic concerns about a possible revival of Japanese imperialism should carefully read the introductory essay by the historian Peter Duus, which admirably sums up the literature.
Japan's takeover of Manchuria in 1931-32 was the doing of a coalition of pro-expansionist domestic forces, led by the military, whose conflicting demands were resolved not by mutual concession but mutual inclusion. Practically no imperialist demand went unmet. Thus instead of pursuing a manageable and minimal expansionist program as did the Meiji oligarchs, the leaders pursued a maximal and unmanageable one. A number of factors contributed to the demands for and ideology of expansionism, including growing resource dependency, perceived overpopulation, and the breakdown of free trade. After the takeover, unlike the Meiji oligarchs, who knew that Japan was no match for the Western powers, the leaders became overconfident and reckless, particularly when the West did not respond. Finally, the success of the Nazi armies in Europe in 1940-41 convinced many Japanese leaders that they could form a Eurasian alliance with Germany that would effectively counterbalance Anglo-American power.