William D. Leahy, American admiral and diplomat, United States Ambassador to France (b. 1875)
Fleet Admiral William Daniel Leahy (May 6, 1875 – July 20, 1959) was an American naval officer who served as the senior-most United States military officer on active duty during World War II. He held multiple titles and was at the center of all major military decisions the United States made in World War II.

As Chief of Naval Operations from 1937 to 1939, he was the senior officer in the United States Navy, overseeing the preparations for war. After retiring from the Navy, he was appointed in 1939 by his close friend President Franklin D. Roosevelt as the governor of Puerto Rico. In his most controversial role, he served as the United States Ambassador to France from 1940 to 1942, but had limited success in keeping the Vichy government free of German control.

Leahy was recalled to active duty as the personal Chief of Staff to President Franklin D. Roosevelt in 1942 and served in that position throughout World War II. He chaired the Chiefs of Staff and was a major decision-maker during the war. He continued under President Harry S. Truman until finally retiring in 1949. From 1942 until his retirement in 1949, he was the highest-ranking active duty member of the U.S. military, reporting only to the President. He was the United States' first de facto Chairman of the Joint Chiefs of Staff (not his official title) and he also presided over the American delegation to the Combined Chiefs of Staff, when the American and British staffs worked together.

As fleet admiral, Leahy was the first U.S. naval officer ever to hold a five-star rank in the U.S. Armed Forces. USS Leahy (DLG-16) was named in his honor, as is Leahy Hall, the U.S. Naval Academy admissions office.