Victory in the Spanish-American War left the U.S. with new decisions to make. By the end of 1898, the U.S. had acquired a number of new island territories: Guam, Puerto Rico, and the Philippines, ceded to the U.S. in the Treaty of Paris; Cuba, which the U.S. Army had governed for four years; and Hawaii, which the U.S. annexed in 1898 independent of the Spanish-American War. In establishing U.S. governing policies abroad, Congress faced two pressing questions: first, how to grant Cuban independence in a way that would also protect American interests on the island; and second, whether or not to annex the Philippines.

In 1901, the Platt Amendment enumerated the conditions for the U.S. Army’s withdrawal from Cuban soil. The amendment required Cuba to vow to make no treaty with a foreign power, to limit its independence, and reserved for the U.S.:

Though the Cubans did not like the restrictions on Cuban sovereignty, they did accept the amendment. The U.S. held wide powers over Cuba for more than thirty years, and maintains its military base to this day.

In the Senate, proponents of expansionism won the debate about the Philippines. Influenced by business interests who saw the Philippines as a valuable gateway to China, the Senate voted to annex the country rather than give it independence. Filipino rebels resisted U.S. rule by attacking the U.S. base of operations, setting off two years of fighting that finally ended with a U.S. victory. The Philippines remained a part of the United States until 1946, when the U.S. granted it independence.

The U.S. government aimed to promote U.S. business and open trade markets in China. China’s Manchu Dynasty was weak and particularly vulnerable to foreign intervention, as evidenced by the spheres of influence that other nations—Russia, Germany, France, England, and Japan—had succeeded in carving out. These nations had each secured exclusive trading rights to certain key ports in China, so that entire regions, or spheres, were blocked to U.S. business. In 1899, as a way to open up all “exclusive” ports to American business, Secretary of State John Hay proclaimed an Open Door policy in China, which meant that no favoritism would be awarded at Chinese ports. European countries, however, refused to endorse this policy. In the following years, Hay continued working to secure advantages for U.S. firms as part of his policy of economic expansionism, which sought not to control new territory but rather to open new markets.

The extreme influence that European nations and the United States exerted in China angered many Chinese. This anger exploded in 1899 in the form of the Boxer Rebellion. In this revolt, an antiforeign secret society calling itself the Harmonious Righteous Fists, known as the Boxers to westerners, killed thousands of foreigners and Chinese Christians and captured Beijing (Peking) in 1900. The U.S. sent 2,500 troops as part of an international force that marched on Beijing and drove out the Boxers. By helping dispel the Boxer threat, the U.S. secured some bargaining power in the settlement that followed. Hay demanded that an Open Door policy be implemented in all of China, and other powers agreed. The Boxer Rebellion had weakened the Chinese government and by the end of the uprising, the U.S. government committed itself to aiding China’s government in the interest of maintaining open markets for the U.S. in the Far East.

Not all Americans supported American imperialism. In November 1898, after the fighting had ended in the Spanish-American War but before any treaty had been signed, an organization known as the Anti-Imperialist League arose in the U.S. The league opposed American expansion and foreign involvement on the grounds that the U.S. had no right to force its will upon others and also because such involvement would likely incite further conflict. The group had many illustrious members, including the writer Mark Twain and the philosopher William James. In 1899 the anti-imperialists had nearly succeeded in preventing the Senate from ratifying the expansionist Treaty of Paris. This time, however, the forces of imperialism won out, and the Anti-Imperialist League lost whatever strength it might have had.

In September 1901, President McKinley was shot by an anarchist named Leon Czolgosz. Vice President Theodore Roosevelt became president, and continued to implement an aggressive foreign policy. His presidency marked the beginning of the Progressive Era.