Offense, Defense, and the Causes of War

the Causeso f War 1 Is war more likely when conquest is easy? Could peace be strengthened by making conquest more difficult? What are the causes of offense dominance?' 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 warcausingeffects (summarizedin 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 difficult; 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 fiercely. 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 (explanationE). (6)Statesmore often adopt fait accompli diplomatic tactics, and such tactics more often trigger war (explanation F). (7)Statesnegotiateless readily and cooperatively;hence negotiations fail more often, and disputes fester unresolved (explanationG). (8)States enshroud foreign and defense policy in tighter secrecy, raising the risk of Stephen Van Evera teaches international relations in the Political Science Department at the Massachusetts Institute of Technology. Thanks to Robert Art, Charles Glaser, and an anonymous reviewer for their comments on this article. It is distilled from Causesof War,VolumeI: The Structure of Power and the Roots of War (Ithaca, N.Y.: Cornell UniversityPress, forthcoming1999). 1. In this article"offense dominant" means that conquestis fairly easy; "defense dominant" means that conquestis very difficult.It is almostnever easierto conquer than to defend, so I use "offense dominant"broadly, to denotethat offense is easierthan usual,although perhapsnot actuallyeasier than defense.I use "offense-defense balance" to denote the relativeeaseof aggressionand defense against aggression. As noted below, this balance is shaped by both military and diplomatic/ political factors.Twomeasuresof the overalloffense-defense balance work wek (1) the probability that a determinedaggressorcould conquerand subjugatea target statewith comparableresources; or (2) the resource advantage that an aggressor requires to gain a given chance of conquering a target state. I use "offense" to refer to strategic offensive action-the taking and holding of territory-as opposed to tactical offensive action, which involves the attack but not the seizure and holding of territory. International Security, Vol. 22, No. 4 (Spring1998),pp. 5-43 0 1998 by the President and Fellowsof HarvardCollege and the Massachusetts Instituteof Technology. 5 International Security 22:4 I 6 miscalculation and diplomatic blunder (explanation HI. (9) Arms racing is faster and harder to control, raising the risk of preventive wars and wars of false optimism (explanation I). (10) Offense dominance is self-feeding. As conquest grows easier, states adopt policies (e.g., more offensive military doctrines) that make conquest still easier. This magnifies effects 1-9 (explanation J). The perception of offense dominance raises these same ten dangers, even without the reality. If states think the offense is strong, they will act as if it were. Thus offense-defense theory has two parallel variants, real and perceptual . These variants are considered together here. How does this theory perform in tests? Three single case-study tests are performed below. They corroborate offense-defense thee$ and indicate that it has large theoretical importance: that is, shifts in the offense-defense balance -real or perceived-have a large effect on the risk of war. The actual offense-defensebalance has marked effects; the effectsof the perceived offensedefense balance are even larger. What causes offense and defense dominance? Military technology and doctrine , geography, national social structure, and diplomatic arrangements (specifically , defensive alliances and balancing behavior by offshore powers) all matter. The net offense-defense balance is an aggregate of these military, geographic, social, and diplomatic factors. How can offensedominance be controlled?Defensivemilitary doctrines and defensive alliance-making offer good solutions, although there is some tension between them: offensive forces can be needed to defend allies. Offense dominance is more often imagined than real, however. Thus the more...