Both wired and wireless communication saw their first real military testing during the bloody World War I, especially on the long-stalemated Western Front. The early battles of the Marne in the west and Tannenberg in the east underscored the importance of good communications. But after the war of movement ended in September 1914, armies limited their use of tactical radio, the equipment for which was still cumbersome to use. Only late in the war did forward units obtain field radio equipment. Germany experimented with radios for its airship fleet, as did both sides, late in the war, with airplanes, first sending messages from ground to air, and then both ways.

Military radio in 1914 was crude on both sides of the conflict. Antennas were obvious targets, and equipment was fragile, cumbersome, and vulnerable to weather or enemy action. There were few trained operators and never enough radios available (a U.S. Army division of 20,000 men rarely had more than six radios even in 1918). But radio’s biggest drawback was the lack of senior commanders willing to use or trust it in battlefield conditions. Poorly organized at first, Army radio users also suffered from security breaches such as sending vital messages in the clear rather than in code. One concern was that all radio signals were subject to being heard by the enemy and thus required effective systems of message coding. To allow short-range telephony with little chance of being overheard, the British introduced the use of the Fullerphone in trench warfare.

While all sides sought to “listen in,” the British most effectively developed the direction-finding receivers and careful traffic analysis essential to successful code breaking. German undersea cables were cut by the British in the early days of the war, forcing the enemy to use radio transmissions to which the British could tune—and eventually understand as their code-breaking expertise expanded. With the help of codebooks seized from captured German naval vessels, the Royal Navy Intelligence, or Room 40 cryptanalysis staff, was able to decrypt many German naval signals— including the infamous “Zimmermann telegram” urging Mexico to declare war on the United States, which finally brought the United States into the war in early 1917. Until the end of the war, however, cryptography remained poorly integrated with operational practice. American efforts, for example, some under the Army’s Herbert O. Yardley, were only partially successful.

World War I naval forces also made extensive use of radio to control widely dispersed fleets. In the 1916 battle of Jutland (and in many other battles), admirals often failed to make the best use of radio information, relying on flag signals that might be misread in battle conditions. As sparkgap equipment was replaced (1916–1917) by better arc and then (1918) vacuum tube–powered equipment, naval radio’s value increased further. Wartime needs and growing equipment procurement greatly accelerated the pace of radio’s technical development. Vacuum tube–based equipment, rare in 1914 (when obsolete spark-gap wireless telegraphy was still widespread), was becoming standard by 1918, vastly increasing radio’s capabilities by adding voice to code communication. Until 1916, German U-boats, too cramped to carry bulky long-wave radio equipment, were limited to shorter-range (200–300 miles) radio links. As vacuum tube technology made possible longer-distance sending and receiving, submarines shifted their attacks farther into the Atlantic. Not all ships’ captains appreciated their loss of independent action with the development of wireless.

More than radio, telegraph and telephone lines linked fighting units down to the battalion level. Some of the 38,000-mile telephone service by 1918 was designed and operated by AT&T on behalf of the military; Army Signal Corps personnel operated the remainder. Hello Girls acted as operators to allow more men to be assigned to military duties. Because lines could be so easily broken in the fighting, however, effective command and control often depended on the use of couriers (frequently mounted on horses, bicycles, motorcycles, or small motor vehicles) or message-carrying pigeons or dogs, as in the past. One carrier pigeon, for example, Cher Ami, helped to get messages through that led to the rescue of the famous “Lost Battalion.” Static trench warfare on the Western Front also required widespread use of pyrotechnic signals (such as signal rockets) and whistles to shift troops into or out of trenches or warn of gas attacks.

A new element in military communications and fighting first appeared in this war—propaganda and psychological warfare. “Propaganda” is a type of military communication designed to weaken an enemy before and during operations. It seeks military gains without, or more usually in support of, military force. While used well before nineteenth-century warfare (there are many historical references to propaganda-like combat efforts, and both sides in the American Civil War made use of propaganda), propaganda and psychological warfare really came into their own during the two world wars. Drawing on growing understanding of persuasive techniques—and fear—propagandists for both the Central Powers and the Allies used a variety of communication media to soften up enemy forces and countries. In past times as well as more recent wars, propaganda has drawn on occult themes. These would be greatly expanded in World War II—as would jamming of enemy radio transmitters to try to obliterate their messages.

The Army Signal Corps expanded fiftyfold as it served growing American forces in Europe. This growth created a huge need for trained personnel as well as a formal research and development establishment, so the corps created what would become Fort Monmouth, New Jersey, which remained the country’s chief signal school until 1974. Extensive training programs were established in most countries that introduced principles of wireless (and wired systems) to thousands of men. These trained personnel would play an important role in helping to push radio developments in the years to come.