This study investigates the relationship between depth and agility using the historical method, identifies principles which govern those relationships, and seeks to apply these principles to NATO's current posture. Historical cases used include the German defenses at Kharkov (I), Kursk (II) and Normandy, the Soviet defense at Kursk (I), the Allied defense in the Ardennes, and the Japanese defense in Manchuria. The study concludes that the relationship between depth and agility centers on time. The greater the depth the greater the amount of time to respond. It is further evident that, even in cases of greater relative depth, a certain minimum level of agility is required to capitalize on that advantage or it will ultimately be lost. It also follows that a force lacking in relative depth must be more agile in order to respond successfully to potentially decisive breakthroughs. Here, too, there exists a minimum level. When the force reaches a point that, in spite of its agility advantage it can neither hold the shoulders of a penetration nor form a viable operational reserve, it is so lacking in depth that it cannot succeed. In analyzing NATO's present situation the study finds that, due to political, economic, and technological constraints, NATO has reached a point of diminishing marginal returns in increasing its depth on the battlefield. Although greater depth is desirable, it may not be feasible to achieve it. Increases in agility offer a viable option to this dilemma for the following reasons: 1. Agility is largely a mindset, as is stated in FM 100-5 and shown in the historical cases studied. Training and war games alone should therefore provide a significant improvement. 2. Gains in depth have been the priority for several years and the easy and inexpensive discoveries have probably already been made. 3. Agility has had little recent emphasis and, therefore, could provide some feasibility alternative very rapidly and inexpensively. The study concludes that the