In 1890, naval Captain Alfred T. Mahan published a book entitled The Influence of Sea Power Upon History in which he emphasized the importance of naval power in maintaining national strength. He argued that a nation, in order to have a strong navy, must have ports and colonies throughout the world where their fleet could anchor safely. These colonies and foreign ports would also benefit the home nation with trade. By the 1890s, the United States had already been expanding outward to Alaska and Hawaii. Mahan's book took this expansionism a step further to advocate imperialism. This imperialist sentiment was further amplified by Rudyard Kipling's notorious poem "The White Man's Burden," which claimed that the Caucasian people were the protectors of the world under God and that it was their duty to lead others towards the light of democracy. With both Mahan's and Kipling's beliefs in mind, fewer men were more ardent imperialists than Theodore Roosevelt. Throughout his presidency, he sought colonies and ports all over the world to exert American influence.

The end of the Spanish-American War brought new problems to the United States–namely what to do with the colonies that had been liberated from Spanish rule. On the one hand, imperialists within the government argued that America's interests would be best served in keeping the former Spanish colonies as American colonies. The Philippines, they argued, would serve as both a trading port for American interests in Asia and a military base from which to project power. They also argued that if the United States failed to claim the Philippines as its own, another European country would simply conquer the island after the American troops left. On the other hand, the anti- imperialists–those who felt the United States should promote independence and self-determination for all peoples throughout the world–argued that retaining the Philippines countered the entire reasoning behind fighting the Spanish in the first place. In the end, the Senate chose to annex the Philippines. Feeling betrayed by the United States, Filipino guerillas led by Emilio Aguinaldo, who had originally fought alongside the American forces against the Spanish, turned and waged a bloody insurrection against American occupation that lasted several years. Ironically, more Americans died fighting the guerillas in the Philippines than fighting in the Spanish-American War. The Philippines issue was never fully resolved until the nation was granted independence in 1946.

Similar controversies ensued over former Spanish territories in the Caribbean, particularly the islands of Puerto Rico and Cuba. U.S. troops occupied Puerto Rico after the war until 1900, at which time troops were recalled and the U.S. Senate decided to annex Puerto Rico as a U.S. territory. Fortunately for American forces, the Puerto Ricans did not resist as much as the Filipinos. The Foraker Act established the territorial government on the island, creating a legislature called the House of Delegates and an island Governor and Executive Council that were chosen by the President of the United States. Although Cuba was granted political autonomy, the Senate inserted the Platt Amendment in Cuba's new constitution, which authorized the United States to intervene in the affairs of Cuba anytime it felt necessary to maintain political stability. The amendment also hindered Cuba's foreign relations by stating that the U.S. had to approve of all Cuban treaties with other countries. These provisions effectively destroyed any true independence and subjugated Cuba to American control.

With the Philippines now annexed, the United States began seeking Asian markets to buy U.S. goods. In the late 1800s and early 1900s, China had been defeated in a series of wars and was consequently being divided by the victors, namely Japan, Great Britain, France, Germany, and Russia. To assert its newfound power in Asia, the United States, led by President McKinley and his imperialist Vice President Roosevelt and Secretary of State John Hay, instituted the Open Door Policy, which declared that all nations–not just the European powers–had the right to colonize China. Initially, most of the victors refused to acknowledge this policy until the United States sent troops to China to help France, Germany, and England quell the Chinese Boxer Rebellion against the colonial powers. Once the rebellion had been eliminated, Europe looked more favorably on the United States and allowed American access to Chinese markets.

The United States also exerted its influence in Central America when President Roosevelt "took" Panama, at that time a rebellious province of Colombia. Since the 1850s, the United States had dreamed of building a canal in Central America to link the Atlantic and Pacific Oceans. Until Roosevelt's presidency, however, the only serious effort to build a canal had ended in failure. Determined to make those dreams a reality, Roosevelt initially offered the Colombian government an offer of ten million dollars for a 100-year lease on Panama. When Colombia refused, Roosevelt secretly sent money to hire Panamanian mercenaries to revolt and declare independence from Colombia. The bloodless rebellion took place in November 1903; only one person died. The rebels immediately declared themselves the government of the Republic of Panama and within days signed a treaty allowing the U.S. to begin construction of the canal. Colombia, a poor nation with little military might, could do nothing. In later years, Roosevelt was heavily criticized for his use of unethical methods in obtaining Panama. At the same time, however, it was difficult for many to find fault for Roosevelt's achievement of the rather lucrative impossible dream.

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