Realistic Caution and Ambivalent Optimism: United States Intelligence Assessments and War Preparations Against Japan, 1918–1941

Throughout the years prior to the outbreak of the Pacific War, the United States defence establishment held an ambiguous view on Japanese policy and strategic aims. A number of factors precluded a clear-cut forecast, among the most important of which was the opportunistic and secretive manner in which Japanese leaders formulated their plans. Under the circumstances, the available intelligence could not provide a definite indication of the moves which the Imperial Japanese Navy (IJN) and Imperial Japanese Army (IJA) would undertake. The situation was further complicated because reliable pieces of evidence revealed Japan did not possess the military and economic resources to defeat a coalition of several Great Powers. The Americans were thus not inclined to expect the Imperial forces to undertake a full-scale conquest of the Asia–Pacific region. The inadequate knowledge of Japanese war plans, in turn, was one of the key factors which led United States defence officials to believe that efforts to bolster their military strength in the Far East were not necessary.