The term anti-imperialist refers to those who oppose rule by a larger group, such as an empire. Anti-imperialists traditionally oppose colonialism. They do not believe nations should rule outside their established borders.
During the late 1800s and early 1900s, colonialism was widespread throughout the world. Great Britain had colonies established in India, Nigeria, Hong Kong, and dozens of other places around the world. Similarly, the United States established colonies in the Philippines, Guam, and Haiti, among others. Other European nations, such as France and Holland, also had colonies.
Anti-imperialists were sometimes individuals who did not live in European- or American-held colonies. They simply opposed the idea of colonialism as a policy. Most anti-imperialists were, however, those who resided within the colonies. They promoted freedom from foreign powers. For example, many people living in India during British colonialism wanted freedom. They did not want Britain to rule over their nation. Instead, they wanted to elect their own government, independent from Great Britain.