Shall people be more moral during the war? By mining large-scale historical newspaper archives, we provide promising evidence through big data analysis to answer this question. Taking the American Civil War as the scenario, we exploited the historical newspaper database Chronicling America as a corpus with widely-used moral lexicons, and retrace the diurnal dynamics of collective moral motivation of American society from 1849 to 1876. Sufficient statistics revealed that compared with Pre-war and Post-war, the collective moral motivation of the society increased significantly during wartime. Regression analysis suggested the situational sensitivity of moral motivation, i.e., during the first half of wartime the moral motivation boosted while then decreased in the second half when the war entered a stalemate. Moreover, the two component dimensions, agency and communion, also increased during wartime.