After 1900 the American public lost interest in its new colonies, but the United States continued to expand its power in essentially imperialist ways. This was true principally in the Caribbean region, where the creation of formal and informal protectorates characterized American foreign policy in the period after the Spanish-American War. The war had spurred interest in the building of an isthmian canal, which was to be built as a national project; and following Panama's secession from Colombia in 1903, the project became a reality. The great strategic importance of the Panama Canal, thereafter joined with the considerable American stake in Cuba and its direct sovereignty over Puerto Rico, drew the nation further into Caribbean affairs.
Still fearful of European intervention and solicitous of the growing American economic interest in the area, policymakers in Washington viewed the chronic political instability of the Caribbean and Central American nations as an invitation to foreign penetration and an obstacle to local development. The Roosevelt Corollary to the Monroe Doctrine, enunciated by President Theodore Roosevelt in 1904, claimed for the United States an "international police power," which entailed a general right to intervene and keep order in the Western Hemisphere. Not only Roosevelt but his successors, William Howard Taft and Woodrow Wilson, steadily expanded American hegemony in the Caribbean. By World War I, Cuba, Panama, the Dominican Republic, Haiti, and Nicaragua were in some kind of protectorate status, while Puerto Rico remained an outright colony. Actual military interventions occurred in Cuba (1906–1909), Haiti (1915–1934), the Dominican Republic (1916–1924), Nicaragua (1912, 1927–1933), and Panama (intermittently and on a lesser scale).
Besides the use of special treaty relationships and military force, the United States attempted to maintain a "monopoly of lending," under which Caribbean governments would borrow money only in the United States; it also established customs receiverships in several countries, which effectively placed their government revenues under control of the United States. Meanwhile, private enterprise had permeated the region with American investment and business activity, while the one-crop economies of the Caribbean nations made them heavily dependent upon the American market. Thus, the nominally sovereign states of the Caribbean area were subject to American controls, both formal and informal, which made their real status essentially colonial.
After 1898, the United States was also active in the Far East, but its impact was weaker there than in the Caribbean. Faced with a huge and populous China, and competing with most of the other major world powers, American policymakers could not aspire to regional dominance or military solutions. The "dollar diplomacy" of the William Howard Taft administration (1909–1913) attempted to foster American investment in China and to create international financial arrangements, which would impose a Caribbean-style "monopoly of lending" upon the government of China. This attempt to mobilize American economic strength as a diplomatic tool accomplished little in the Far East, however, on account of both the difficulties of the situation itself and the limited interest of the nation's business and financial leaders. The earlier Open Door policy of 1899–1900, therefore, remained the principal basis of policy. It represented little more than an attempt to obtain a general agreement to preserve the existing treaty system of shared control in China, and thus equality of economic opportunity in China for the United States. The policy was not very effective, and the Chinese market never came near to meeting the inflated expectations of the West. In general, the limited objectives and relative ineffectiveness of American activities in the Far East fell well short of real imperialism in this period, although the United States was long a party to the treaty system by which the Western powers jointly had imposed a limited protectorate upon China.