The era of American prominence in world politics, which began with the twentieth century, occasioned reconsideration of traditional attitudes regarding foreign entanglements and the use of force. From a foreign perspective, the United States was too powerful to ignore but too unpredictable to deal with satisfactorily. From the American viewpoint, global interests and capabilities made for uneasiness and confusion. The new circumstances of American foreign affairs seemed to require reevaluation of traditional American policies and portended new military necessities.
Americans found themselves caught up in new difficulties rather suddenly; they had not fully appreciated the problems that policies and developments of the last decades of the nineteenth century had brought. They had traded as they always had, vigorously and aggressively. They had expanded their boundaries, sought naval strength (beginning in the 1880s), and acquired a position unchallengeable in the hemisphere—but they had not anticipated the effects of such changes on their position beyond the hemisphere. They had hoped to exert a considerable influence on the nations of the world, but by example rather than by forcible instruction. American strength was meant to make the nation impervious to the caprice or malice of European governments and to enable it to vindicate rights and protect interests, but not to require U.S. participation in European political affairs.
Citizens of the United States took pride in the energy, growth, and strength of their nation. Only after they had acquired great power did they begin to consider the problems that national strength created. One idea was appealing: Could not the issue be resolved if the United States were to use force only in just causes, and thus make it the servant of morality? In such terms, Americans preferred to explain their "splendid little war" with Spain—a war, they said, to end Spanish tyranny and repression in Cuba, where the situation of the people had become intolerable. But the liberation of Cuba coincided with the "benevolent assimilation" of the Philippines. The brutal suppression of an insurrection by Filipinos who sought independence rather than assimilation was neither splendid nor little, and it sparked an anti-imperialist protest against what was termed an immoral, un-American abuse of power. The American anti-imperialists failed to alter U.S. counterinsurgency policy in the Philippines. But some of the issues they raised about the legitimacy of the use of force in pursuit of foreign relations objectives would arise again later in the century.
At the turn of the twentieth century, Theodore Roosevelt could confidently add his corollary to the Monroe Doctrine's assertion of the separation of the New World from the Old by proclaiming the right of the United States to wield "police power" in the hemisphere to correct either incompetence or wrongdoing. For Roosevelt and those who shared his vision of America's role in the world, righting wrongs in Latin America and elsewhere ultimately would be determined by American power.
Participation in World War I even more clearly demonstrated the determination to make power and force serve good ends. After two and half years of neutrality, during which he attempted to negotiate a "peace without victors," President Woodrow Wilson called upon Americans to embark on a crusade to make the world safe for democracy. In that war to end all wars, the United States was not content to defeat enemies but fought for principles embodying the liberal features of American diplomacy. So, at least, Wilson explained his conversion from opposing force in 1914–1916 to calling for force without stint in 1917.
By the end of World War I, many Americans were no longer convinced that the United States could ensure that good intentions in the use of force would bring right results. Except for the defeat of the Central Powers, none of the things for which Americans had fought seemed to have been achieved. The keystone of Wilson's fourteen-point peace plan, the League of Nations, was rejected by the U.S. Senate as a result of a combination of partisan politics and Wilson's physical incapacity following a massive stroke. Lacking participation by the most important world power, what has been called the Versailles system of world politics was, in the view of some historians, fatally flawed from its beginning. Others have argued that the peace Wilson sought might have endured but for the actions of revisionist powers such as Germany, Italy, and Japan. The lessons of Versailles and the failure of the League of Nations were even less clear to those who guided American foreign relations between the world wars.