In recent years, the phenomenon of global warming and its implications for the future of the human race have been intensively studied. In contrast, few quantitative studies have been attempted on the notable effects of past climatic changes upon human societies. This study explored the relationship between climatic change and war in China by comparing high-resolution paleo-climatic reconstructions with known war incidences in China in the last millennium. War frequencies showed a cyclic pattern that closely followed the global paleo-temperature changes. Strong and significant correlations were found between climatic change, war occurrence, harvest level, population size and dynastic transition. During cold phases, China suffered more often from frequent wars, population decline and dynastic changes. The quantitative analyses suggested that the reduction of thermal energy input during a cold phase would lower the land carrying capacity in the traditional agrarian society, and the population size, with significant accretions accrued in the previous warm phase, could not be sustained by the shrinking resource base. The stressed human-nature relationship generated a ‘push force’, leading to more frequent wars between states, regions and tribes, which could lead to the collapse of dynasties and collapses of human population size. War frequencies varied according to geographical locations (North, Central and South China) due to spatial variations in the physical environment and hence differential response to climatic change. Moreover, war occurrences demonstrated an obvious time lag after an episode of temperature fall, and the three geographical regions experienced different length of time lags. This research also shows that human population increases and collapses were correlated with the climatic phases and the social instabilities that were induced by climate changes during the last millennium. The findings proposed a new interpretation of human-nature relationship in the past, with implications for the impacts of anomalous global warming on future human conflicts.