William Daniel Leahy (/ˈleɪ(h)i/ LAY-(h)ee; 6 May 1875 – 20 July 1959) was an American naval officer and was the most senior United States military officer on active duty during World War II; he held several titles and exercised considerable influence over foreign and military policy. As a fleet admiral, he was the first flag officer ever to hold a five-star rank in the U.S. Armed Forces.
An 1897 graduate of the United States Naval Academy, Leahy saw active service in the Spanish–American War, the Philippine–American War, the Boxer Rebellion in China, the Banana Wars in Central America, and World War I. He was the first member of his cadet class to reach flag rank, as the Chief of the Bureau of Ordnance from 1927 to 1931. He subsequently served as Chief of the Bureau of Navigation from 1933 to 1936, and commanded the Battle Fleet from 1936 to 1937. As Chief of Naval Operations from 1937 to 1939, he was the senior officer in the United States Navy, overseeing the expansion of the fleet and preparations for war.