In an attempt to determine the effects of war planning on the behavior of countries in crises and wars, this report analyzes the national-level planning that preceded and shaped the German invasion of the Low Countries and northern France in 1940. As a study of war planning in the 1930s by France, Britain, Belgium, and Germany, it sheds considerable light on the way in which political, financial, and manpower constraints guide the military planning process: Threat assessment played a comparatively minor part in planning. Instead, available resources were the single most important determinant of plans. The situation of a totalitarian nation bent on changing the European status quo opposed by a coalition of democracies offers obvious analogies with present-day NATO. The authors discuss the similarities and differences in the historical and current situations, and draw three types of parallels: conceptual parallels, planning process comparisons, and direct similarities.