Final answer:
The primary argument of American anti-imperialism was that empire building ran counter to American democratic principles.
Step-by-step explanation:
The primary argument of American anti-imperialism was the concern that empire building ran counter to American democratic principles.
Many prominent Americans formed the Anti-Imperialist League in 1898 to oppose the expansionism of the United States. They believed that acquiring colonies overseas went against the principles of democracy and freedom upon which the country was founded.
The reasons for their opposition varied, including fear of competition from foreign workers, fear of the United States suffering a foreign invasion, concerns about the integration of other races, and the belief that empire building contradicted American democratic principles.