William Daniel Leahy (/ˈleɪhiˌ ˈleɪ.i/) (May 6, 1875 – July 20, 1959) was an American naval officer who served as the most senior 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 of the U.S. during World War II. As fleet admiral, Leahy was the first U.S. naval officer ever to hold a five-star rank in the U.S. Armed Forces. He has been described by historian Phillips O'Brien as the "second most powerful man in the world" for his influence over U.S. foreign and military policy throughout the war.