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United States

A union of former colonies itself, the United States approached imperialism differently from the other Powers. Much of its energy and rapidly expanding population was directed westward across the North American continent against English and French claims, the Spanish Empire and Mexico. The Native Americans were sent to reservations, often unwillingly. With support from Britain, its Monroe Doctrine reserved the Americas as its sphere of interest, prohibiting other states (particularly Spain) from recolonizing the newly independent polities of Latin America. However, France, taking advantage of the American government's distraction during the Civil War, intervened militarily in Mexico and set up a French-protected monarchy. Spain took the step to occupy the Dominican Republic and restore colonial rule. The Union victory in the Civil War in 1865 forced both France and Spain to accede to American demands to evacuate those two countries. America's only African colony, Liberia, was formed privately and achieved independence early; Washington unofficially protected it. By 1900, the U.S. advocated an Open Door Policy and opposed the direct division of China.

After 1898 direct intervention expanded in Latin America. The United States purchased Alaska from the Russian Empire in 1867 and annexed Hawaii in 1898. Following the Spanish–American War in 1898, the US added most of Spain's remaining colonies: Puerto Rico, Philippines, and Guam. Deciding not to annex Cuba outright, the U.S. established it as a client state with obligations including the perpetual lease of Guantánamo Bay to the U.S. Navy. The attempt of the first governor to void the island's constitution and remain in power past the end of his term provoked a rebellion that provoked a reoccupation between 1906 and 1909, but this was again followed by devolution. Similarly, the McKinley administration, despite prosecuting the Philippine–American War against a native republic, set out that the Territory of the Philippine Islands was eventually granted independence. In 1917, the U.S. purchased the Danish West Indies (later renamed the US Virgin Islands) from Denmark and Puerto Ricans became full U.S. citizens that same year. The US government declared Puerto Rico the territory was no longer a colony and stopped transmitting information about it to the United Nations Decolonization Committee. As a result, the UN General Assembly removed Puerto Rico from the U.N. list of non-self-governing territories. Four referendums showed little support for independence, but much interest in statehood such as Hawaii and Alaska received in 1959.

The Monroe Doctrine was expanded by the Roosevelt Corollary in 1904, providing that the United States had a right and obligation to intervene "in flagrant cases of such wrongdoing or impotence" that a nation in the Western Hemisphere became vulnerable to European control. In practice, this meant that the United States was led to act as a collections agent for European creditors by administering customs duties in the Dominican Republic (1905–1941), Haiti (1915–1934), and elsewhere. The intrusiveness and bad relations this engendered were somewhat checked by the Clark Memorandum and renounced by President Franklin D. Roosevelt 's " Good Neighbor Policy ".

The Fourteen Points were preconditions addressed by President Woodrow Wilson to the European powers at the Paris Peace Conference following World War I. In allowing allies France and Britain the former colonial possessions of the German and Ottoman Empires, the US demanded of them submission to the League of Nations mandate, in calling for V. A free, open-minded, and absolutely impartial adjustment of all colonial claims, based upon a strict observance of the principle that in determining all such questions of sovereignty the interests of the populations concerned must have equal weight with the equitable government whose title is to be determined. See also point XII.

After World War II, the U.S. poured tens of billions of dollars into the Marshall Plan, and other grants and loans to Europe and Asia to rebuild the world economy. At the same time American military bases were established around the world and direct and indirect interventions continued in Korea, Indochina, Latin America (inter alia, the 1965 occupation of the Dominican Republic), Africa, and the Middle East to oppose Communist movements and insurgencies. Since the dissolution of the Soviet Union, the United States has been far less active in the Americas, but invaded Afghanistan and Iraq following the September 11 attacks in 2001, establishing army and air bases in Central Asia.