Final answer:
Alfred T. Mahan was the naval theorist and historian who defined sea power and advocated for a strong and expansive U.S. Navy, which influenced the country's foreign policy and military strategy.
Step-by-step explanation:
The person who defined sea power, highlighted the importance of understanding naval needs, and advocated for a large, powerful Navy was Alfred T. Mahan, a naval theorist and historian.
Mahan's seminal work, 'The Influence of Sea Power upon History, 1660-1783,' published in 1890, underscored the significance of naval power in national prosperity and global influence. Through his teachings and influence on figures such as Theodore Roosevelt, Mahan argued for a modern naval fleet equipped with big ships and big guns, acquisition of strategic coaling stations globally for refueling, and the establishment of naval bases.
His work encouraged the United States to expand and modernize its navy, positioning it as a formidable maritime force by the end of the 1890s.
Mahan's ideas were not just instrumental in the American context but also influenced other world powers, leading to a naval arms race that contributed significantly to global military tensions and eventually the outbreak of the World Wars. His emphasis on the development of a powerful navy to protect and extend American commerce and influence was a cornerstone for American foreign policy and military strategy at the turn of the 20th century.