The rejection of the Treaty of Versailles by the Senate and the overwhelming popular ratification of that action in the election of 1920 can be regarded as a triumph of American isolationism. It was not, as has sometimes been argued, a return to an earlier policy. The world had changed too much to allow that. But it was a reassertion of that policy in the face of the first fundamental challenge it had ever faced. The isolationism of the 1920s was real, despite the continuing commercial expansion of the United States and despite the greater influence on world affairs that the country enjoyed. The traditional policy, which the isolationists thought they were preserving, had always, after all, emphasized trade and commerce even while shrinking from political commitments, and American influence and the desire for it had traditionally been a component of the "mission" of the United States.
Nevertheless, the American position during the 1920s was in some ways an ambiguous one. The experience of World War I had greatly increased the role of the United States as an economic, political, and even military factor in world affairs, and made some degree of coordination with other nations all the more crucial. At the same time, the war had served as an object lesson on the danger of international commitments. The success of the Bolshevik Revolution in Russia was only the most threatening of the postwar events that persuaded most Americans that their intervention had clearly failed to make the world safe for democracy. It thus appeared to demonstrate the wisdom of the contention that meddling in the affairs of others was useless and self-defeating. The reflexive logic that this intervention had almost led to a total abandonment of the policy of the Founders only served as a further warning.
On the basis of such perceptions, the United States set out on an isolationist course that could best be described as one of cooperation without commitment. The United States, for the first time in its history, sharply curtailed immigration. It took the lead in negotiations on naval disarmament that would make war less likely and took pains to clarify the purely hortatory character of the Open Door policy. The Four-Power Treaty of 1921 changed any commitments the United States might once have assumed with respect to the openness or territorial integrity of China into a commitment, proposed by Senator Lodge, "to communicate … fully and frankly in order to arrive at an understanding as to the most efficient measures to be taken, jointly or separately, to meet the exigencies of the particular situation." Even then, the Senate ratified the treaty only after adding a further disclaimer: "The United States understands that under the statement in the preamble or under the terms of this treaty there is no commitment to armed force, no alliance, no obligation to join in any defense."
During these years the most heralded diplomatic achievement by the United States, the Kellogg-Briand Pact (1928), was in its origins simply a way of gracefully denying France the security guarantees it had sought to obtain. Although generally regarded at the time as a positive contribution to the maintenance of world peace and order, it formally committed the United States to no action of any kind and was strongly supported by many of the most prominent isolationists, including the new chairman of the Senate Foreign Relations Committee, Senator Borah.
By the beginning of the 1930s, the United States was retreating from military intervention in Latin America by adopting the so-called Good Neighbor Policy, and Secretary of State Henry L. Stimson reacted to the Japanese conquest of Manchuria with a unilateral action that threatened nothing more serious than nonrecognition. President Herbert Hoover restated the isolationist consensus in 1931. Although recognizing a greater interdependence among nations in the modern world, Hoover nonetheless distinguished between the path of the United States and that of other nations. "We should cooperate with the rest of the world;" he told his cabinet, "we should do so as long as that cooperation remains in the field of moral pressures…. But that is the limit." There were few dissenters.