Electronic interceptions appeared as early as 1900 during the Boer War of 1899 to 1902. The British Royal Navy had installed wireless sets produced by Marconi on board their ships in the late 1890s.
The birth of signals intelligence in a modern sense dates from the Russo-Japanese War of 1904 to 1905. A report from HMS Diana on Russian Signals intercepted at Suez dated the 28th of January 1904 exists in the Naval library at the Ministry of Defence in London.
In 1941, Ultra exerted a powerful effect on the North African desert campaign against German forces under General Erwin Rommel. Official historian Sir Harry Hinsley argued that Ultra shortened the war by not less than two years and probably by four years.
The United States Department of Defense has defined signals intelligence as a category comprising communications intelligence, electronic intelligence, and foreign instrumentation signals intelligence however transmitted. The two main sub-disciplines are communications intelligence and electronic intelligence.
Rear Admiral Henry Oliver appointed Sir Alfred Ewing to establish an interception and decryption service at the Admiralty known as Room 40. Captain H.J. Round began carrying out experiments with direction-finding radio equipment for the army in France in 1915.