Nationalism, an effort to exert greater influence beyond the country's borders, and a willingness to threaten or use force, while not new in United States foreign policy, seemed more apparent as the country moved into the twentieth century. Several episodes of American foreign and military policy highlight this trend.
Deficiencies of the U.S. Army in the Spanish war necessitated a revamping of the military organization. Using European precedents, Secretary of War Elihu Root proposed several changes, including creation of the Army War College and a general staff. Much opposition came from entrenched interests in the army and the state militias, but through compromise Root's proposals passed. America's participation in World War I was more effective because of these changes. For some people development of the general staff raised a specter of militarism. Walter Millis, a student of militarism, writing in 1958 commented on Root's contribution and mused that without it American participation in the Great War might not have occurred. "But Root, like all large figures," Millis said, "was only a reflection of his times. There were many other architects of the great disaster of militarism which was to supervene in 1914–18."
The new navy was begun under the administrations of James A. Garfield and Chester A. Arthur. In 1883, Congress approved four steel vessels, and the building program continued through subsequent administrations, especially the Mahan-influenced presidency of Theodore Roosevelt, when emphasis was on battleship construction. A major turn came after war broke out in Europe. Since Roosevelt's presidency, American naval policy had called for a navy second only to Great Britain. In 1915 policy proclaimed a navy second to none. The naval appropriations act of 1916 had no precedent for its naval construction plans. A strong opponent, House majority leader Claude Kitchin, argued futilely that approval would make the United States "the greatest military-naval Nation the world has ever seen." The act reveals an interesting dichotomy, showing the uneasy American attitude toward military measures by combining large appropriations for warships with a renunciation of armed aggression, and an endorsement, in principle, of disarmament. Wilson's support for a strong navy shows his realization of the interaction of military power and diplomacy. The navy would allow the United States to meet existing challenges and to perform the international tasks it expected after the war.
In Latin American policy and in implementation of the Monroe Doctrine, Americans showed a new assertiveness resulting, particularly in the twentieth century, in frequent military interventions, intervention to remain a standard response to political instability until the 1930s. In 1896, Secretary of State Richard Olney and President Grover Cleveland confronted the British with "the United States is practically sovereign on this continent, and its fiat is law upon the subjects to which it confines its interposition." A few years later, Theodore Roosevelt, fearful of European intervention (perhaps a German naval base in the debt-ridden Dominican Republic), accepted for the United States the role of international policeman in the Caribbean. From the Roosevelt Corollary, the Platt Amendment with Cuba, the responsibilities of dollar diplomacy, and the 1903 canal treaty with Panama, whose independence Roosevelt assured by timely naval maneuvers, there emerged a Caribbean foreign policy often characterized by the big stick. American troops and tutelage countered political and economic chaos. Clashes were bound to occur: in 1912, U.S. forces in the Caribbean for the first time went into battle to suppress revolutionaries, this time in Nicaragua. In ensuing years, the Dominican Republic, Haiti, Mexico, and Nicaragua experienced extensive interventions, often with violence and with a full-scale guerrilla war in Nicaragua in the 1920s and early 1930s. Guerrilla opposition was not new to Americans, who had faced it in the Philippines after the Spanish-American War.
If broad interpretation of the Monroe Doctrine in areas near the Panama Canal and if growing interests in Far Eastern affairs born of the Open Door and the search for that vast Asian market cast the United States in a role of greater involvement calling for more reliance on military solutions, this was not accompanied by surrender of traditional attitudes toward military matters. Compromises were always necessary: even entering the war in 1917 was put in the perspective of fighting German militarism, of fighting a war to end war. While most Americans might applaud a combative nationalism that had Roosevelt proclaiming, "Perdicaris alive or Raisuli dead"— when Raisuli, a Moroccan bandit, abducted an alleged American citizen near Tangier—or while pacifist Jane Addams might lose much of her popularity during World War I and have her speeches considered dangerous, or while Eugene Debs might be sent to jail in 1918 for a speech condemning war, these and similar events were more a result of the exuberant patriotism of the times than a widespread tendency toward militarism. In the postwar years there is substantial evidence that Americans wished to return to policies less likely to involve force. Much of the opposition to the League of Nations came from those who thought Article X of the Covenant deprived Congress of a free hand in deciding on war. These men did not want to guarantee the "territorial integrity and existing political independence of all members of the League," for that might lead to a war not in America's interest. The disarmament conferences and the Kellogg-Briand Pact to outlaw war as an instrument of national policy did not usher in a long era of peace, but they were symptomatic of peaceful desire. Weary of the Nicaraguan imbroglio, President Herbert Hoover and Secretary of State Henry L. Stimson concluded that marine interventions were too costly and should end. Revelations of war profiteering, exposés of the armament industry, and the revisionist historical literature on U.S. entry into the Great War brought disillusionment and a determination that it should not happen again. Presidential power in foreign policy was suspect, the neutrality laws of the 1930s tried to close all loopholes that might lead to war, and there were restraints on presidential flexibility in foreign policy. One proposal, the Ludlow Resolution (1935–1938), even indicated distrust of Congress on the matter of war by urging that declarations of war should be by national referendum. These questions became more pressing as world crises multiplied. During the debate on American foreign policy in the late 1930s and early 1940s, each side proclaimed its approach as the true road to peace while the opponent's was sure to involve the United States in war.