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Assessing the Conventional Balance The 3:l Rule and Its Critics I John 1.Mearsheimer I T h e conventional balance in Europe has lately become the focus of intense debate. Until now participants in this debate have concentrated on the substance of the issuewhether NATO forces can defend successfully-while leaving aside methodological questions. Specifically, although analysts use widely different methods, there has been little debate about which methods are most reliable. Recently, however, Joshua Epstein published a criticism of others’ work that raises important methodological questions that merit close scrutiny.’ One of Epstein’s principal criticisms is directed against the well-known and widely accepted 3:l rule of thumb.*This rule posits that the attacker needs a local advantage of at least 3:l in combat power to break through a defender’s front at a specific point. Epstein purports to offer historical evidence that disconfirms the 3:l rule. The 3:lrule applies to just one aspect of a campaign, namely, breakthrough battles in which the attacker attempts to pierce the defender’s forward defenses . It does not bear on other important events that affect the outcome of the campaign. Specifically, it is not relevant to analyzing each side’s ability to concentrate forces at main points of attack before and during the breakthrough battle, and it has little relevance to the attacker’s ability to execute a deep strategic penetration following a breakthr~ugh.~ Among the many people who offered helpful suggestions and criticismsI would especially like to thank Christopher Achen, Robert Art, Richard Betts, Stephen Biddle, Daniel Bolger, Henry Brady, Charles Glaser, Ted Greenwood, John Lepingwell, Barry ONeill, Barry Posen, Lawrence Samuels, Jack Snyder, Marc Trachtenberg, Stephen Van Evera, and Stephen Walt. Van Evera coauthored the sections of the article that explicate the 3:1 rule and its uses. john Mearsheimer is a Professor in the Political Science Department at the University of Chicago. He is the author of Conventional Deterrence (19831 and Liddell Hart and the Weight of History (1988). 1. See Joshua M. Epstein, ”Dynamic Analysis and the Conventional Balance in Europe,” International Security, Vol. 12, No. 4 (Spring 1988), pp. 154-165. 2. In this article Epstein also criticizes other approaches to conventional net assessment. His four principal targets are “bean counters,” proponents of the 3:l rule, the Attrition-FEBA Expansion Model devised and explicated by Richard Kugler and Barry Posen, and proponents of Lanchesterian approaches, such as William Kaufmann. 3. A synopsis of these and other factors that would decide a European campaign is found in International Security, Spring 1989 (Vol. 13, No. 4) 01989 by the President and Fellows of Harvard College and of the Massachusetts institute of Technology 54 The 3:l Rule I 55 However, breakthrough battles would be crucial in a European war, and NATO’s success would ride on its ability to win these battles. Many analysts use the 3:l rule to assess these battles; the rule thus underpins many assessments of the European conventional balan~e,~ and is central to many estimates of NATO force requirements and evaluations of conventional arms control proposals. Hence if Epstein’s criticisms of the 3:l rule were correct, they would have important policy implications. His criticisms, however, are not valid. When the 3:l rule is evaluated against the proper evidence it appears quite reliable. Epstein’s case against the rule rests on misuse of a historical data base which is itself flawed. Epstein also fails to report better historical data, presented by Colonel Trevor Dupuy in one of the studies that Epstein relies upon. Had he presented this data his case would largely have collapsed. Epstein also claims that he has developed a model for breakthrough battles that overcomes the deficiencies of the 3:l rule and the other models he criticizes. An examination of Epstein’s model, however, indicates that it is seriously flawed, as is the manner in which he has used it to measure the conventional balance in Europe. His analysis rests on a false picture of a modern armored offensive, and the tactics available for defense against it. Epstein and I are in broad agreement on the state of the...


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