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When one hears the term "American Empire" usually the first impressions in one's mind are so-called neoconservative policies, military adventures, and academic-style bashing of American history and policy. Historians meanwhile may think back to the post-Spanish-American War era and the time of Savage (Small) Wars for Peace. There also was a peaceful effort to expand the United States past its continental restrictions: this effort of the mid to late 1800s was centered on guano.
Guano, excrement primarily from birds, was the early nineteenth century's wonder resource. This oil of its day was great for fertilizer and the manufacturing of gunpowder. Guano was so highly valued that Chile took the "horrible" Atacama Desert from Bolivia in the War of the Pacific because of the rich quantities of guano. The United States also highly valued the guano so much that it passed 1856 Guano Islands Act that allowed private citizens to officially claim territory for America; this was a risky bill because it created international disputes that could have easily led to war with any number of foreign powers.
Most of the claimed islands were in the South Pacific but several were also in the Caribbean Sea. Some islands were inhabited by natives and others were claimed by various states like the United Kingdom and France but most were desolate rocks and reefs that only were valuable because of the large historic bird guano mounds they featured. Today most claims have been formally abandoned by the United States in various treaties. The Republic of Kiribati is a primary beneficiary of American withdrawal. However, some key places like Midway remain in American control and have featured prominently in the country's history.
The dream of an American Guano Empire existed more on map then in reality. However, the United States managed to hold onto the various claims it would have established a sphere of influence in the Pacific and Caribbean that would have dominated both major bodies of water. Japan would be pressed against the Asian Coast, the United Kingdom would only have Greater Australia and New Zealand, and France would be stuck in the southern most rim of the Pacific Ocean.
The late nineteenth century saw the rise of chemical science and the downfall of the value of guano. Claims remained on paper but the United States was unwilling to enforce many of them. The dream of a Guano Empire quickly switched to the American Expansion of the Spanish-American War.