Such ideas became even more prevalent in the aftermath of the war as Wilson sought to develop a new global system, based on the Open Door rather than traditional colonialism. The keystone of the president's new program was to be the League of Nations, a body of the world's governments that would ensure "collective security" by taking action against aggressor states, militarily if necessary. Immediately a large and diverse coalition of critics came forth to condemn this departure from America's isolationist ideology, as they saw it. Some politicians, led by Senator Henry Cabot Lodge, an old-line Republican from Massachusetts who represented northeastern commercial interests, feared that the league would damage U.S. sovereignty, forcing America to participate in collective action at the behest of other members of the new organization. "Are you willing to put your soldiers and your sailors," he asked, "at the disposition of other nations?" Missouri Senator James Reed invoked a sense of racial superiority, charging that "black, brown, yellow and red races," ranking low in "civilization" and high in "barbarism," would be on equal footing with the great, white United States.
Lodge and Reed, however, were not specifically opposed to the extension and use of American power, but many others were, and saw danger in the league. La Follette believed that it would become an "imperialist club" that would maintain the status quo and keep colonies such as Ireland and India in bondage because the new body was not likely to sanction action against the great powers that held sway over the less-developed world. Like La Follette, others such as senators Borah, Hiram Johnson, and George Norris were so-called irreconcilables, who were progressive on domestic matters and believed that the league not only would limit American autonomy but also would deny autonomy to poor nations, and was not consistent with traditional national virtues of self-determination and isolation from the intrigues and squabbles of Europe and elsewhere. Further, a broad consensus was emerging to question America's involvement in and future after World War I; it was feared that the United States was embarking on a path of global behavior, with entanglements and interests abroad, which would resemble that of the existing empires. Indeed, in the years during and just after the war, a number of anti-imperialist and antimilitarist groups—including the Fellowship of Reconciliation, the American Friends Service Committee, the War Resisters League, and the Women's International League for Peace and Freedom—emerged to lobby for a more insular and less aggressive foreign policy. In the face of such widespread criticism, Wilson held his ground and refused to negotiate or compromise with his detractors, and the Senate accordingly rejected the treaty to join the League of Nations. While the war had marked America's debut as a great world power, the United States would not don the trappings of empire.
That is not to say, however, that the United States retreated from world affairs. Although the period between World Wars I and II is usually referred to as one of isolationism, the 1920s, as the historian and anti-imperialist Charles Austin Beard remarked, saw a "return to the more aggressive ways … to protect and advance the claims of American business enterprise." Trade and investments, and intervention, abroad increased between the wars as a corporative alliance of government offices and business institutions sought to create order and stability at home as well as to establish such conditions outside of national boundaries. In addition to reestablishing and augmenting economic ties to a rebuilding Europe and pressing for a greater opening of Asian markets, U.S. officials and corporations continued to move into Latin America in pursuit of expanded business opportunities.
Such circumstances led to another wave of anti-imperialism in the 1920s and 1930s but, once more, in complicated and seemingly contradictory ways. American officials such as secretaries of state Charles Evans Hughes and Frank Kellogg, concerned about exorbitant military spending and the potential for another outbreak of hostilities, brokered international agreements on disarmament and to outlaw war as an instrument of national policy. They and their successor Cordell Hull believed that free trade would promote peace, whereas empire led to conflict. Isolationists in public life and the media also believed that Europe was still trapped in the type of rivalries that had caused war in 1914, and warned that the United States should stay clear of foreign engagements until that continent stabilized. Such critics, however, were often internationalists who did not question America's right or need to expand abroad, but saw contemporary conditions as a deterrent to foreign involvements at that time.
Others offered a more pointed analysis. Critics of the war and the League of Nations treaty, such as La Follette and Borah, continued to warn against American imperialism and militarism, and spoke out against U.S. attempts to crush nationalist liberation movements in Nicaragua and El Salvador. Marine General Smedley Butler became something of a folk hero and offered a compelling critique of American imperialism when he called himself a "racketeer, a gangster for capitalism." During his thirty-three years in the Marine Corps, Butler boasted, he had
helped make Mexico and especially Tampico safe for American oil interests in 1914. I helped make Haiti and Cuba a decent place for the National City Bank boys to collect revenues in. I helped in the raping of half a dozen Central American republics for the benefit of Wall Street. The record of racketeering is long. I helped purify Nicaragua for the international banking house of Brown Brothers 1909–12. I brought light to the Dominican Republic for American sugar interests in 1916. I helped make Honduras "right" for American fruit companies in 1903. In China in 1927 I helped see to it that Standard Oil went its way unmolested…. Looking back on it, I feel that I could have given Al Capone a few hints. The best he could do was to operate his racket in three districts. I operated on three continents.
Butler's views gained widespread acceptance. Huey Long, governor of Louisiana and putative presidential candidate when assassinated in 1935, agreed with the general and promised to nominate him to be secretary of (anti) war if elected in 1936. In fact, throughout the 1930s, disillusioned with World War I and alarmed by revelations and charges from the senate's Nye Committee that corporations, particularly in the munitions industry, had lobbied, if not conspired, for entry into the Great War, a majority of Americans held isolationist positions. Decrying what Senator Gerald Nye had termed the "rotten commercialism" of American businesses during the war years, Congress, with public support, passed a series of neutrality acts and other measures to prohibit President Franklin Roosevelt from becoming involved in conflicts in China, Ethiopia, and Spain.
The continuing aggression of Nazi Germany and imperial Japan, culminating in the attack on Pearl Harbor on 7 December 1941, however, undercut the anti-intervention, anti-imperial consensus and set the United States onto a path of apparently irreversible global empire. By war's end in 1945 the United States stood as the world's only great power: as a condition for aiding Britain during the war, the United States had insisted on the opening up of the markets of the empire and the beginning of a process of decolonization; Germany and Japan were in ruin as a result of the fighting that laid waste to Europe and Asia; and the principal rival to American hegemony, the Soviet Union, had lost more than 20 million people and millions of farms and factories during the war. The United States controlled half the world's trade and had established an economic order, the Bretton Woods system, and a political institution, the United Nations, as means to wield its power and influence. The so-called American century was in full bloom but, U.S. leaders warned, without a permanent military establishment and arms buildup it would be in constant peril. Accordingly, the United States embarked on its greatest military expansion, began to establish a global network of bases, sought an international Open Door, and established a national security state at home.