Following World War I, the United States reaffirmed unilateralism as its preferred modus operandi on the world stage. From 1921 to 1941 the debate between Harding-style nationalism and Wilsonian internationalism continued. Nationalism held sway for twenty years, transforming itself in the 1930s into the doctrine of isolationism. Foreign policy was nationalist in the sense that important matters were settled not by treaties but by the unilateral acts of the United States (either declarations by the executive branch or, more commonly, laws passed by Congress). In 1921 and 1924 laws settled the immigration question through the institution of quotas (a matter normally involving relations with the other countries concerned). Laws were passed to raise tariff rates (Fordney-McCumber Tariff of 1922 and Smoot-Hawley Tariff of 1930). A law announcing U.S. adherence to the Permanent Court of International Justice was voted on 27 January 1926, but it was accompanied by so many reservations that the World Court ultimately rejected America's candidacy.
The United States acted unilaterally in Asia in the case of the Stimson Doctrine. Following the Japanese invasion of Manchuria in late 1931, Secretary of State Henry L. Stimson issued identical notes to Japan and China on 7 January 1932, asserting that the United States would not recognize any impairment of American treaty rights in China and morally condemning Japanese aggression. The Stimson Doctrine was later (March 1932) endorsed by the League of Nations. Other unilateral decisions included the Neutrality Acts (1935–1937), which imposed embargoes on arms and munitions destined for all belligerents, whether aggressors or victims. A promise of independence for the Philippines took the form of a law (Tydings-McDuffie Act of 1934). There were declarations broadening the application of the Monroe Doctrine. An exchange of letters sufficed for recognition of the Soviet Union in 1933. Everything seemed to indicate that Americans, after having rejected the greatest treaty of the century, had become suspicious of treaties, especially of collective ones. Nonentanglement no longer extended to alliances only, but also to numerous other types of treaties.
However, despite rejection of the multilateral Versailles approach, the United States under certain conditions was very willing to sign collective agreements. The clearest cases emerged from the Washington Conference of 1921–1922: the Five-Power Naval Disarmament Treaty (6 February 1922), the Nine-Power Treaty on China (6 February 1922), the Four-Power Treaty on the Pacific (13 December 1921), and the London Naval Conference agreements (1930 and 1935). The United States also forced Japan to renounce its claims to German rights in the Shantung Peninsula and in Siberia.
Another illustration of American willingness to act in concert with other nations was the Pact of Paris (the Kellogg-Briand Pact) of 27 August 1928, signed by an unprecedented sixty-three nations. This pact had originated in the useless, romantic proposal by French Foreign Minister Aristide Briand on the tenth anniversary of U.S. entry into the war (April 1927) that France and the United States undertake never to wage war on each other. At the urging of the chairman of the Senate Foreign Relations Committee, William Borah, Secretary of State Frank Kellogg proposed that the pact be expanded to include all nations. War "as an instrument of national policy" was to be renounced (thereby leaving open the possibility of military sanctions voted by the league). In this way it was possible to satisfy American pacifists like Salmon Levinson, who rejected the League of Nations but called for "the outlawry of war," as well as men like James Shotwell and the Carnegie Endowment for International Peace, who sought an alternative to collective security. There was no provision for enforcement or sanctions, however, and the Japanese violated the pact in Manchuria as early as 16 September 1931, leaving Stimson with little to fall back on except the moral condemnation contained in his "doctrine."
This was also the period of Pan-Americanism. Whereas the United States made few treaties with Europe or the Far East during these years, it concluded a fairly large number with Latin American republics. Matters such as evacuation of occupied Caribbean nations were generally settled by executive agreements (Dominican Republic, 1922; Nicaragua, 1927); but many pacts were signed at the conferences of Havana (1928), Montevideo (1933), Buenos Aires (1936), and Lima (1938). The most important were those of Montevideo, stipulating that "No State has the right to intervene in the internal or external affairs of another," and Lima, providing for consultations between ministers of foreign affairs whenever there was danger of war.