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Beginning in the late 1880s, a new assertiveness characterized American foreign policy. In 1893, Americans in Hawaii forced the islands’ Queen Liliuokalani to abdicate. The United States annexed Hawaii five years later. War with Spain in 1898 led to the acquisition of Puerto Rico, Guam, and the Philippines, where the United States confronted a two-year insurrection.

Fear that the United States was being shut out of trade with China led Secretary of State John Hay’s 1899 Open Door Note, which sought to guarantee equal access to Chinese markets and to prevent imperial powers from carving China into spheres of influence and establishing special concessions.. The Roosevelt Corollary to the Monroe Doctrine declared that the United States would exercise “international police power” in the Western Hemisphere. The United States assisted Panama in securing its independence from Columbia in order to build a canal across the Isthmus of Panama. The U.S. occupied Nicaragua for twenty years, Haiti for nineteen years, and the Dominican Republic for eight years.