In the Treaty of Paris that formally ended the war, Spain granted the United States Cuba, Puerto Rico, and Guam, and McKinley graciously agreed to buy the Philippines for $20 million. The United States did eventually honor the Teller Amendment and withdrew from Cuba in 1902, but not before including the Platt Amendment in the Cuban constitution, establishing a permanent U.S. military base at Guantánamo Bay.
The war gave McKinley more headaches than it cured. First, McKinley was faced with an insurrection when Emilio Aguinaldo turned against American forces in the annexed Philippines. It took several years of jungle warfare before the insurrection was put down, but even then, Filipinos resisted assimilation into American culture.
Second was the problem of what to do with all the new people in the territories America had taken over. In 1901, the Supreme Court ruled in the Insular Cases that people in newly acquired foreign lands did not have the same constitutional rights as Americans living in the United States. Congress nonetheless upheld the 1900Foraker Act that granted Puerto Ricans limited self-government and eventually full U.S. citizenship by 1917.
Finally, McKinley had to contend with the new, vocal Anti-Imperialist League and its prominent membership. The league challenged McKinley’s expansionist policies and the incorporation of new “unassimilable” peoples into the United States.