United States had had unique tradition of supporting local volunteer militia and avoiding large scale standing army, ‘the Anti-Military Tradition,’ since colonial time till the early 20th century. After experiencing the record increase of economic power and population, United States changed in large scale in all aspects, and it finally turned from the isolation of the agricultural republic to become an empire true to the name especially after the advent of Mckinley administration and Spanish American War. United States Army Reform became possible when the new political groups who recognized the new status of their country in the international system cooperated with the reform minded officer corps raised after the Civil War. The turning point of army reform was made by the appointment of Elihu Root as a new Secretary of War who had much political acumen and knew very well the long existing political split lying in the problem of army reform. Based on these recognition, Root accomplished a series of fundamental reform of army institutions through leading public opinion by using various media, searching for harmony with traditions, providing new logics to overcome traditions and negotiating and compromising with opposing groups actively. With several compromises, Root reform could not help but have considerable limitations and defects, the reform experienced the period of recession and stalemate from 1904 to 1909 also with continued oppositions based on anti-military tradition. The army reform had another turning point when the general Leonard Wood, who had long been raised up by reform minded group, became the chief of staff of army general staff. The general staff, which was the centric and symbolic organization of army reform, could function normally according to the vision of reform group at last, providing several documents embodying the concrete vision of new army. Once the world war broke out in Europe in 1914, the debate surrounding the army reform entered into the new phase with the increased attention of public for war preparedness. The time period from 1914 to 1916, which can be labeled as the period of reform completion, was the time during which reform group finally succeeded in persuading American public in general the social importance of army in democracy and the necessity of enlarging and organizing the army and mobilization system. The training camp movement and war preparedness movement clearly showed this process, and the reform opposing group finally accepted much of reform policy. In 1916, when the Wilson government lost initiative while facing the increasing threat from Germany, Democratic Party in the Congress made efforts to pass the law to prepare United States for the unexpected emergencies arising from Europe, trying not to lose leadership to Republican. The result, the National Defense Act of 1916, which was the very first whole scale law regulating all areas of defense issues, included many aspects which reformers had long wished for so it could be estimated as the final accomplishments of reform efforts since 1899, although having some problems at the same time.