Nations at War provides a scientifically-derived explanation of war. It develops this explanation by reviewing data-based studies of international conflict, analyzing war from the fifteenth to the twentieth centuries, and identifying factors such as geography, regimes, military capabilities, alliances, and trade associated with both the onset and destructiveness of these conflicts. Two wars (the Iran/Iraq war of 1980, and World War I) are examined in detail in an effort to show how wars begin and sometimes expand to include other states. This unique book collates and synthesizes the findings of over five hundred scientific studies of war.