By 1898, the United States already exerted influence over Hawaii, which it formally annexed that year five years after American business leaders deposed the native monarch and established a republic, in which no native Hawaiians held power. Republicans generally supported this action, seeing the commercial and strategic advantages of establishing American power in the Pacific.
Also that year, growing American sympathy for Cuban rebels seeking independence from Spain, as well as the USS Maines explosion in Havana harbor, led the United States to declare war on Spain on 25 April 1898. The American decision to take the Philippines was based on the same economic and strategic motives. The United Kingdom, France, and Germany already claimed colonies or other influence in eastern Asia and the Pacific, and the United States used the war as an opportunity to claim its own by annexing the Philippines and ruling them until 1946.
Though President McKinley and others claimed they took the Philippines because the Filipinos were not yet civilized enough for self-rule, economics and politics were the true motive, and McKinley himself claimed that doing otherwise would have been bad business and discreditable.
Davis, Kenneth C. Dont Know Much about History. New York: Avon, 1990. Goldfield, David et al. The American Journey. Upper Saddle River NJ: Pearson, 2005. Henretta, James A. et al. Americas History. Boston: Bedford/St. Martins, 2008.