In terms of the sheer scale of human suffering involved, Pearl Harbor was a minor episode by comparison with what happened to the peoples of Japanese-occupied Asia -- China, Korea, Indonesia, the Philippines, etc. At the time of Pearl Harbor, the Japanese aggression against China had already been underway for a decade, while Japan's colonial occupation of Korea and Taiwan was already almost half a century old. The Western powers had few qualms concerning these earlier phases of Japanese imperialism. In fact, in the early part of the twentieth century British ruling class observers had expressed great admiration for Japan's colonial exploits (in tsarist Russia they shared a common enemy).
Observing the world around them in the mid-nineteenth century, Japan's rulers realized that they faced a simple choice -- to fall victim to the Western colonial powers, like their Chinese neighbors, or to exert the herculean effort needed to learn from them and become like them. Alone among the underdeveloped countries, Japan set its sights upon imitating the existing colonial powers and succeeded in becoming a colonial power itself. Despite a few exotic paraphernalia like emperor worship, imperial Japan modeled itself on its Western colonial precursors. Like them, it took pride in its technological modernity and boasted of its "civilizing mission" to the "backward" peoples of the world.
If Japan learned from its Western teachers, later the Western powers also learned from Japanese "achievements." Wartime Japan had an intensive biological warfare research program -- the infamous Unit 731, which conducted horrendous experiments on prisoners of war (Chinese, Russian, and several other nationalities). After the war, the US military authorities gave all those involved in Unit 731 immunity from prosecution in exchange for access to the results of their experiments -- information that the US used to develop its own biological warfare capability. During the Korean War the US, advised by former members of Unit 731, conducted bacteriological warfare against northeast China. At the time these charges were dismissed as communist propaganda, but the meticulous research of Stephen Endicott and Edward Hagerman has established beyond reasonable doubt that they were true (The United States and Biological Warfare: Secrets from the Early Cold War and Korea, Indiana UP, 1998).
Another massive atrocity of imperial Japan was the kidnapping and enslavement of Korean and other "comfort women" to serve as prostitutes to Japanese soldiers. See, for example, George Hicks, The Comfort Women: Japan's Brutal Regime of Enforced Prostitution in the Second World War (W.W. Norton, 1994). Here again we find a certain continuity between the Japanese militarists and their American victors and successors. In Chapter 6 of his book, entitled "The end of a nightmare, the beginning of another," Hicks recounts how the "comfort women" system continued under the US occupation of Japan. The victims now were Japanese women, enlisted from the lower classes by brothel keepers with help from the civil and military authorities to serve American soldiers, thereby "protecting" Japanese women of the upper class. True, in this case it was not necessary to kidnap women on a large scale, though there was some use of force: usually the threat of starvation was a sufficient goad.