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Offense, Defense, and the Causes of War

Is war more likely when conquest is easy? Could peace be strengthened by making conquest more difacult? What are the causes of offense dominance?1 How can these causes be controlled? These are the questions this article addresses. I argue that war is far more likely when conquest is easy, and that shifts in the offense-defense balance have a large effect on the risk of war. Ten warcausing effects (summarized in Figure 1) arise when the offense dominates. (1) Empires are easier to conquer. This invites opportunistic expansion even by temperate powers (explanation A). (2) Self-defense is more difacult; hence states are less secure. This drives them to pursue defensive expansion (explanation B). (3) Their greater insecurity also drives states to resist others’ expansion more aercely. Power gains by others raise larger threats to national security; hence expansionism prompts a more violent response (explanation C). (4) First-strike advantages are larger, raising dangers of preemptive war (explanation D). (5) Windows of opportunity and vulnerability are larger, raising dangers of preventive war (explanation E). (6) States more often adopt fait accompli diplomatic tactics, and such tactics more often trigger war (explanation F). (7) States negotiate less readily and cooperatively; hence negotiations fail more often, and disputes fester unresolved (explanation G). (8) States enshroud foreign and defense policy in tighter secrecy, raising the risk of