If we take the definition of Imperialism to mean a policy of extending a country's power and influence through diplomacy and/or military exercise, then from colonial days the country has engaged in such a policy by destroying Native American nations as the European population increased. Certainly the Monroe Doctrine suggested the US dominate the New World, whether it was able to do so at that time or not. However, after the Civil War, what ultimately became the contiguous United States was under one political system, and internal Imperialism changed to external Imperialism, first in the Caribbean, then to Central and South America, and the island nations in the Pacific. Many of these where remnants of the Spanish Empire, and without much effort the US exerted control, going to war in 1898 against Spain and fighting in Cuba and the Philippines. Part of what also drove the imperialist expansion was the desire for new resources to feed the expanding American industrial base, which had expanded during and after the Civil War.