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The process of territorial expansion, which had been highly politicized in the 1840s' "Manifest Destiny" campaigns, was stalled by the political revolution of the 1850s and the subsequent Civil War and Reconstruction. Of approximately twenty opportunities to acquire overseas territory between 1865 and 1889 only six were seized. Around 1890, however, American expansionist tendencies regained strength. Of the twenty-five opportunities to acquire territory between 1890 and 1908, twenty-three resulted in some form of expansion.

Scholars continue to debate the reasons behind the shift. Some see an implicit desire to validate the emerging American self-image as a world power. This theory is tied to the concept of "classical realism," the idea that states expand their power and influence when they have the opportunity to do so, as the Europeans had been doing for centuries. Other scholars cite "defensive realism"; if the United States did not act, her vital interests might be jeopardized. Still other scholars see economic motives and the growing importance of international commerce behind the resurgent expansionism of the late nineteenth and early twentieth centuries. The uncertainties in American policy become apparent when one recognizes that certain areas long coveted by some Americans, like Cuba, were not annexed when they might have been, yet other areas, like the Philippines, were seized after the briefest of national discussions. Regardless of theories, it is clear that U.S. expansion developed in a broader context of expanding European empires throughout the world.

Overseas expansion after 1890 also paralleled the growing influence of the American navy. The most important American strategist in this period was Alfred Thayer Mahan (1840-1914), head of the Naval War College and author of The Influence of Sea Power upon History (1890); he was admired by the Prince of Wales, the German Kaiser, and Theodore Roosevelt. Mahan, like the Greek historian Thucydides, argued that the survival of any great power depended on a strong navy, and that a strong navy depended on island possessions that could serve as naval bases.

This module focuses on the two primary areas of American expansion: Middle America including the Caribbean; and the Pacific . The pattern of expansion varied considerably. In some cases, the United States engineered the purchase of new territory (as in the case of Alaska); in other areas American "interests" undermined the local government and asked for annexation (as in the case of Hawaii). In at least one instance (Samoa), the native government sought American protection.