Following tours as the executive officer of two battleships, Ingersoll headed the branch of naval intelligence responsible for code-breaking efforts against the Japanese. In 1927, he graduated from the Naval War College and was promoted to captain. He then held staff assignments and commanded cruisers. For three years, from 1935 to 1938, Ingersoll headed the War Plans Division, where he helped revise Plan Orange against Japan. In 1937, he launched informal discussions with Great Britain on potential Anglo-American cooperation in any future conflict with Japan.
Promoted to rear admiral in 1938, Ingersoll commanded Cruiser Division 76. He was recalled from sea duty in 1940 and became assistant to Chief of Naval Operations Admiral Harold R. Stark. Ingersoll played a crucial role in helping prepare the U.S. Navy for war. In January 1942, as a vice admiral, he took command of the Atlantic Fleet, based at Norfolk, Virginia; his mission was to counter the German U-boat campaign, thereby safeguarding Atlantic lines of communication and protecting convoys bound for Europe, and to secure the Western Hemisphere. In June 1942, his only son and namesake, a naval lieutenant, died in action at the Battle of Midway. Ingersoll was promoted to full admiral the following month. That November, his vessels transported the U.S. Western Task Force to Morocco in Operation torch, thenceforth supporting Allied operations in the Mediterranean.
From May 1943, Ingersoll worked closely with the new Tenth Fleet, established under the direct command of Chief of Naval Operations Admiral Ernest J. King. Ingersoll's logistical and managerial skills proved particularly valuable in deploying American forces to maximum effect, and, by substantially neutralizing German submarine forces, greatly facilitated the June 1944 Normandy landings.
In November 1944, Ingersoll became commander, Western Sea Frontier, to implement the complex transfer of American naval forces from the Atlantic to the Pacific for the projected invasion of Japan, a task effectively obviated by the sudden end of the war in August 1945. He retired one year later. Ingersoll died in Washington, D.C., on 20 May 1976.
Hughes, Terry, and John Costello. The Battle of the Atlantic. New York: Dial, 1977.; Macintyre, Donald. The Naval War against Hitler. New York: Scribner's, 1961.; Morison, Samuel Eliot. History of the United States Naval Operations in World War II. Vol. 5, The Struggle for Guadalcanal. Boston: Little, Brown, 1949.