Alfred Thayer Mahan (September 27, 1840 – December 1, 1914) was a United States Navy flag officer, geostrategist, and historian, who has been called "the most important American strategist of the nineteenth century." His concept of "sea power" was based on the idea that countries with greater naval power will have greater worldwide impact; it was most famously presented in The Influence of Sea Power Upon History, 1660–1783 (1890). The concept had an enormous influence in shaping the strategic thought of navies across the world, especially in the United States, Germany, Japan and Britain, ultimately causing the World War I naval arms race. His ideas still permeate the U.S. Navy Doctrine.
Several ships have been named USS Mahan, including the lead vessel of a class of destroyers.
Mahan's views were shaped by the seventeenth century conflicts between Holland, England, France and Spain, and by the nineteenth century naval wars between France and Britain, where British naval superiority eventually defeated France, consistently preventing invasion and blockade (see Napoleonic war: Battle of Trafalgar and Continental System). To a modern reader, the emphasis on controlling seaborne commerce is commonplace, but in the nineteenth century, the notion was radical, especially in a nation entirely obsessed with expansion on to the continent's western land. On the other hand, Mahan's emphasis of sea power as the crucial fact behind Britain's ascension neglected the well-documented roles of diplomacy and armies; Mahan's theories could not explain the success of terrestrial empires, such as Bismarckian Germany. However, as the Royal Navy's blockade of the German Empire was a critical direct and indirect factor in the eventual German collapse, Mahan's theories were vindicated by the First World War.
In the context of his time, Mahan backed a revival of Manifest Destiny through overseas imperialism. He held that sea power would require the United States to acquire defensive bases in the Caribbean and Pacific as well as take possession of Hawaii. This came at the time when the United States launched a major shipbuilding program to move the United States to the third place amongst worldwide naval powers by 1900.