2.1.3. War

War data were gathered from the Tabulation of Wars in Ancient China, an appendix that belongs to the Military History of China, which was summarized by the Editorial Committee of Chinese Military History [34] and has been extensively employed in previous research [1–5,8–10,14–20]. In this study, battle was considered the basic unit of war. The criterion is that if two sides engaging in a war have a fight in reality, then such a fight is regarded as a battle. The terms 'battle' and 'war' are interchangeable in this study when referring to different categories. All the ancient battlefields in this compendium were verified using the Historical Atlas of China [35], counted within the present territory of China, and converted into currently used place names. In other words, battlefields beyond the national boundary were excluded even though they were historically in the areas that belonged to the Chinese Empire. The exclusion of such battlefields may affect the spatial pattern of war, ye<sup>t</sup> these outliers only account for a small proportion of all records—they are usually difficult to locate due to extreme remoteness and the lack of documents. Thus, 5501 battlefields during 1–1911 CE were identified in this study. This number only represents battle locations with definite coordinates, which means the actual number is larger. However, battles do not actually occur at an exact point. From a micro perspective, a battle should have a combat zone, which cannot be measured because people never know much about it. Therefore, the hot spots derived from battle points or coordinates can only be examined in a large framework. The spatial scale of this study was set to national rather than regional and local scales. The definitions of different kinds of wars (i.e., agri-nomadic conflict and rebellion) are provided in Supplementary Materials.
