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The volume in the series Senshi-sosho entitled Headquarters of the Imperial Army I holds the view that the imperial defense policy (IDP) was hardly changed by its first revisions and its nature and policy was maintained. Research conducted after the publication of "Senshi-sosho" also basically reinforced this view. This paper argues that the IDP was fundamentally and drastically revised by its first revisions in 1918. The strategic thought behind the original IDP was formulated on the supposition of a short and limited war against one single country either Russia or the U.S.. Therefore, the Army and Navy requested 50 divisions in wartime and 2 fleets consisting of 8 battleships and 8 cruisers. On, the contrary, the revised IDP was devised on the assumption of a long, all-out war against the U.S., Russia and China at the same time from the lessons Japan learned during WWI. The Army upgraded the strategic force from "divisions" to "corps", and the necessary size was determined as 41 corps in all-out war, and the Navy requested. one more fleet of 16 battleships and 8 cruisers. What changed the nature of the IDP on such a large scale ? "The Necessity of a National Mobilization Plan" proposed by vice-chief of staff Tanaka Giichi was approved in 1917. It assumed that Japan would fight a war against several nations simultaneously and denied the strategic concepts based on the lessons from the Russo-Japanese war. It was proposed to make a defense policy that desired a short and limited war due to limited resouces, but also understood that Japan may have to fight a long and enduring all-out war. In its "Explanation of the Budget" to the Ministry of Finance in 1919 in accordancey with the new IDP, the Ministry of the Army requested funds for the construction and maintenance of a main force of 22 corps in peace time and 41 corps in wartime to fight an all-out war against several enemies. The idea of creating corps-oriented forces, however, was not realized; and the force was returned to the 40-42 divisions structure in 1920, because of popular opposition to any kind of enlargement of the army, the financial crisis caused by the depression, and disagreement inside the Army over how to organize an all-out war posture. The first IDP revision had two significant points. First, the concept of "National Mobilization Posture" made clear the vulnera-bility of Japan's limited resources and low manufacturing capacity. Therefore, a consensus was built up not only within the military, but also among politicians, that Japan should seek its insufficient resources in China and build a self-sufficiency structure. As a result, the strategic area of the new IDP was expanded to all of East Asia, including mainland China. As more and more Japanese made their way into China, the U.S.-Japandde rivalry grew more fierce, and the possibility of Anglo-Japanese confrontation heightened. Soon Japan became internationally isolated. Secondly, ideological confrontation over the posture fot an all-out war emerged within the military. Tanaka Giichi and Ugaki Kazushige found an all-out war posture essential and advocated a transformation of the army, while Uehara Yusaku and Fukuda Masataro regarding an early stage of war as vital, insisted on the maintenance of the status quo. This rivalry continued into the Showa era.