As late as the beginning of 1940, with World War II several months old, military liaison between Canada and the United States was so scant that they had not even exchanged service attaches. Yet the two countries and their armed forces were inevitably brought into extensive and intimate collaboration in the prosecution of World War II. This study is a historical record of the military and politico-military aspects of this collaboration. The impact of advancing technology since World War II on time and space factors has demonstrated ever more forcefully that the defense problem of the two countries is a continuing one requiring joint solutions. In consequence, the two countries have in recent years been drawn into even closer co-operation. This study is intended to provide background information to staff officers currently involved in defense planning, to officers on exchange duty with the Canadian armed forces, and to officers in the service schools preparing for such duties. Since many of the current joint problems are similar to those of World War II, these officers should find in the record of World War II experience guidance which will help them achieve the optimum solutions to their current problems. From the analyses of the politico-military relationships between the two countries, many lessons can be gleaned by Americans of both the military and the diplomatic services, as well as by civilian scholars. The study will perhaps be similarly useful to Canadians. In its broader aspects, the experience recorded herein may be applicable, with interpretation, to similar arrangements between other pairs of neighboring countries or within a multilateral security arrangement. The author of this volume, Col. Stanley W. Dziuban, began work on it early in 1950 to satisfy the doctoral dissertation requirement of Columbia University, from which he received a Ph.D. degree in 1955. Colonel Dziuban, a 1939 graduate of the U.S. Military Academy, is at present assigned as Deputy Division Engineer, U.S. Army Engineer Division, New England.