Read statements from Japanese diplomats rationalizing the brutality of Japanese troops in China.
Professor Rana Mitter explains the origins of the Japanese Pan-Asianism.
Japanese imperialism was about increasing its territory on the Asian mainland and elsewhere in the region, but it wasn't simply fueled by territorial ambition. There was also a strong ideological sense of mission. And this was encompassed in a word, which we don't hear much nowadays but which was very much used at the time, the idea of Pan-Asianism. In other words, in the Japanese mind, the idea that they were expanding their empire in Asia, not simply to increase their own territory but to liberate those territories from Western imperialism—from the British, the French, the Dutch, the Americans—all of whom had colonies in the region.
Now, this idea, of course, was highly misleading because, of course, essentially the Japanese were just replacing one empire with another. But they argued that because they were Asians too, because they had an idea of wanting to bring their Asian brothers to the same level that the Japanese themselves were at, that somehow their mission was different. And even today in Japan in some quarters, you will hear a justification of Japan's military aggression and expansion on exactly those grounds. Pan-Asianism started as an idea that had more to do with shared Asian culture, but by the 1930s, it was really a fig leaf for militarist expansion.