The dissertation explores and illuminates the largely neglected Atlanticist tradition in America that preceded the advent of the Cold War and the creation of NATO. It does so by showing that the concept of an 'Atlantic community' was developed by several intellectuals and scholars starting in the 1890s and was then re-articulated at various moments of crisis during the first half of the 20th century, each time relying on a ampler set of related assumptions. Central to early Atlanticist views was a reflection on the logic of international relations and the sources of world order. The Atlantic community conveyed the vision of an international order built on the core of the 'Atlantic democracies' but capable of expanding globally as additional countries came to accept the political and economic principles of the 'liberal Western tradition'. In so arguing, the dissertation challenges the widespread belief that the vision of an Atlantic West, even in its pre-Cold War dimension, mainly revolved around the anticipation of a struggle between America and Russia and the expectation of American hegemony over Europe. It argues instead that, from its earliest days, the Atlantic community was a much broader worldview, whose aims included the preservation of American democracy in an increasingly interdependent world, the maintenance of peace during the anticipated hegemonic transition between America and Great Britain, and the notion that America's involvement in world affairs must lead to a re-foundation of international relations by transcending the balance of power and establishing some form of world government. In this context, the dissertation examines the evolution of the American Atlanticist tradition from both chronological and thematic points of view. It concentrates on a few turning points when the concept of an Atlantic community enjoyed particular popularity and acquired new relevance and meanings. These included America's 'imperialist moment' in the 1890s, its intervention into World War I, the abandonment of neutrality in 1941, and the formation of the Atlantic Alliance in 1948-1949. Thematically, the Atlanticist tradition is divided into three main strands: realist-navalist, Christian, and federalist Atlanticism - each examined in depth through the writings of its main proponents, including Alfred Thayer Mahan, Walter Lippmann, Francis P. Miller, Reinhold Niebuhr and Clarence Streit. Although each strand had its specific focus and audience, all three of them shared the same underlying assumption: in order to tackle the issue of world order in a way consistent with the preservation of American democracy and the advancement of American power, it was essential to foster cooperation and integration among the nations bordering the Atlantic.