Allied Convoy in the Atlantic, circa 1942, U.S. Navy photograph in the collections of The Mariners' Museum
U-Boat Transmitting Center Ashore in France, circa 1944,
courtesy of Horst Bredow
How Speed Defeats a U-Boat,
Montegue Dawson, Oil on canvas, The Mariners' Museum
 
U-177 (Type IXD/2) Running on the Surface, circa 1942,
courtesy of Horst Bredow
U.S. Navy Signalsmen Monitoring Enemy Radio Transmissions in the Atlantic, circa 1944,
U.S. Navy photograph in the collections of The Mariners' Museum

Assembling merchant vessels to sail in convoy with armed escort vessels is a standard naval defensive tactic that can limit the threat of commerce raiding surface vessels and submarines. During World War II, the Allies used the convoy system to defend large numbers of merchant ships with limited numbers of escorts. German naval leaders countered this defense with radio, marshaling multiple submarines for concentrated attacks against Allied convoys. These "group tactics" (widely known as "wolfpack") were designed to overwhelm Allied escorts, often enabling the attacking U-boats to penetrate the heart of the convoy and sink merchant ships from within.

Relying on radio communications to coordinate vessel movements from onshore headquarters, Axis and Allied naval leaders were both vulnerable to radio "signals intelligence." In coordinating their naval campaign against Allied merchant shipping, German naval leaders inadvertently provided Anglo-American intelligence trackers with a means of pinpointing Axis vessel locations, thus depriving U-boats of submarine stealth and surprise. After 1943, U.S. Navy trackers systematically used signals intelligence first to divert convoys and then to hunt down and kill the would-be Axis attackers. Without Allied convoys safely crossing the Atlantic, the decisive June 1944 "D-Day" invasion of Axis-occupied Europe might have been delayed.

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