Before World War I, British planners recognized that undersea telegraph cables were a major strategic asset because they carried fast, reliable communications across empires. A 1911 British government subcommittee concluded that if war came with Germany, German-owned undersea cables should be destroyed to disrupt command and control and shape the information war.
Hours after Britain entered World War I, the cable ship CS Alert sailed from Dover to locate and cut German submarine telegraph cables in the English Channel. Cutting these lines sharply reduced Germany’s secure overseas cable communications and pushed more traffic onto routes the British could monitor or onto radio, which was easier to intercept.
As Germany’s East Asia Squadron crossed the Pacific, it attacked the British cable relay station on Fanning Island (part of the “All Red Line” cable system). The raid damaged the station and cut undersea cables, showing that cable stations—often isolated and lightly defended—were valuable targets for disrupting long-distance communications.
A landing party from the German cruiser SMS Emden cut the communications cable linking Perth and the Cocos (Keeling) Islands. The cable was later recovered and repaired, but the incident highlighted how a single cut could temporarily isolate a key relay point and trigger rapid repair-and-defense responses.
With German cable routes degraded, British intelligence collected more German traffic traveling over neutral-country cables and over radio. Britain built up its codebreaking and analysis capability in “Room 40,” turning intercepted communications into operational and diplomatic intelligence. This period helped set patterns for later military signals intelligence: disrupt secure links, then exploit what replaces them.
British interception and decryption of the Zimmermann Telegram demonstrated how control of cable routes could shape global politics. Direct German cable transmission was blocked early in the war, and Britain’s ability to capture and decode German diplomatic communications helped influence U.S. public opinion and policy in 1917.
After World War I, major powers treated cable routes, shore stations, and landing points as critical infrastructure with military value. Planning increasingly assumed that wartime communication depended on redundancy—mixing cables and radio—and that both systems could be attacked or exploited. These ideas carried into World War II as undersea systems faced renewed threats.
After Germany occupied the Channel Islands in 1940, it fortified coastal positions and used facilities such as lighthouses for observation and naval reporting. These outposts supported control of nearby sea lanes and created targets for Allied raids aimed at disrupting communications and gathering intelligence.
British commandos raided the Casquets lighthouse and German manned outpost, destroying radio equipment and seizing materials including code books. The raid targeted a small but useful communications-and-observation node, showing how special operations could strike coastal reporting networks tied to maritime security.
Allied raiders attacked the Glomfjord power plant in Norway to disrupt electricity that supported aluminum production, an important war material. While not an undersea-cable attack, it fit the same logic: strike infrastructure that enables enemy communications and industry, then force repairs and rerouting under wartime pressure.
In Italy in late 1943, small British special forces teams combined rail and road demolition with cutting telephone and power lines. These actions aimed to slow troop movement and reduce the speed of German command and reporting. The pattern echoed World War I cable warfare: physical disruption forces the enemy onto less reliable alternatives.
By the end of World War II, both world wars had shown that undersea cables and related shore facilities were not just civilian utilities—they were military objectives and intelligence opportunities. Cutting or seizing communications links could isolate enemies, while intercepted traffic could influence operations and diplomacy. These lessons helped shape the post-1945 security mindset around protecting and monitoring undersea communication networks.
Undersea cable warfare, interception, and sabotage in WWI and WWII (1914–1945)