Between 1941 and 1945, Americans expressed outrage over Japan’s attack on Pearl Harbor and subsequent military aggression. Numerous commentators, citizens, and opinion-makers looked beyond wartime atrocities and regularly vilified Japanese for the crime of “ingratitude.” Japan, they argued, had not merely attacked the country that had opened it to the outside world a century earlier, but had also declared war on the people who had saved its citizens in 1923. This article explores why, amidst the great whirlwind of wartime inhumanity, Americans harkened back to their 1923 humanitarian engagement with Japan following the Great Kantō Earthquake. Many did so, I suggest, to assist wartime mobilization, to lionize America’s righteous global stature, and to forge and reinforce constructions of their enemy’s sub-human character. Only humans, many angry Americans argued, understood or could express feelings of gratitude. Highlighting Japan’s supposed “ingratitude,” and their “betrayal” of America’s humanitarian generosity served as an emotive way to dehumanize all Japanese beyond the well-documented discussions of wartime aggression, treachery, or “innate racial characteristics.” Elites employed these constructions drawn from their enemy’s supposed ingratitude to help legitimate a brutal war waged without mercy against soldiers and civilians alike.