Final answer:
Alfred Thayer Mahan influenced American policy with his focus on naval power, arguing for a strong navy to ensure national greatness, which led to the expansion of the US Navy and global naval power.
Step-by-step explanation:
Alfred Thayer Mahan helped define American policy in the 1880's with his work on the influence of naval power on history. Mahan, a naval theorist and historian, asserted that a nation's greatness and prosperity were closely linked to its naval strength.
His influential book, The Influence of Sea Power upon History, 1660-1783, argued that control over the seas was critical for national security and economic prosperity.
Mahan recommended a strategic expansion of the US navy, which included the construction of modern battleships, the establishment of overseas coaling stations and naval bases, and the acquisition of territories like Hawaii for strategic purposes. His ideas profoundly impacted policy makers, including the rise of the US as a global naval power and the eventual military arms race preceding both world wars.