Picture this: It is an international crisis.  In response to an unprovoked attack on American citizens and property by foreign revolutionaries, American troops have been dispatched into another country to hunt them down.

Accompanying the American forces are special radio units which are intercepting communications that provide the American commander with locations, strength, and movements of the revolutionary forces, as well as the country’s own military forces. With this information, the American forces are able to avoid a major confrontation.

Signal Corps Radio Tractor Van located along the Mexican border – note the early Harleys

Does this scenario sound familiar?  If so, consider your memory a good one, for this happened 80 years ago during the 1916 Pershing Expedition’s pursuit of “Pancho” – Villa’s revolutionary forces in Mexico.  With General Pershing came the U.S. Army’s newly organized Radio Tractor Units, which provided direct, tactical support in the form of what was then called radio intelligence.

The story of the RTU’s began in 1914 when the U.S. Army’s Signal Corps accepted its first three unites.  They were designed to operate in a mobile environment, providing their own power and carrying the accessories necessary to perform the mission of intercept and communications.

General John Joseph “Black Jack” Pershing

The RTU’s first taste of action occurred in 1916.  On March 9, troops of “Pancho” Villa’s revolutionary forces crossed the border and attacked the town of Columbus, New Mexico.  In retaliation, President Woodrow Wilson ordered General John Pershing to lead 6,000 American troops into Mexico in pursuit Villa and his men.  Accompanying Pershing were two RTU’s; a third bivouacked just across the border in Arizona.  From the moment the RTU’s arrived, they provided direct support to Pershing.  Monitoring the radio communications of the Mexican Federal forces, the units were able to keep General Pershing apprised of the movements of the Federal and revolutionary forces (Villa had no radios).

More importantly, from the very beginning of the expedition, the War Department recognized the important of the intercepted intelligence and established a system to distribute it to all interested Army commands and the Department of State.  By the end of the expedition in early 1917, intelligence could be intercepted, decrypted, translated, and distributed within two to three days among recipients in Washington, D.C., Army command in Texas and Arizona, and Pershing’s headquarters in Mexico.  The speed of the system is all the more remarkable when it is remembered that the primary means of communicating the intelligence was wire telegraphy.

A Signal Corps Radio Tractor in the field, circa 1918

By 1917, with America’s entry into World War I, the American Army left Mexico.  Back in the United States, the RTU mission changed.  At first, the units were positioned along the Mexican border to do field hearability studies.  But, with the Zimmennan Telegram’s dramatic revelation of German machinations in Mexico and the United States’ declaration of war on Germany, the mission of the RTU expanded to include intercept of German diplomatic communications with Mexico, as well as the monitoring of the communications among the various German Legations inside that country.

The RTU’s demonstrated their flexibility by adapting quickly to strategic intercept.  By war’s end 12 intercept stations and four direction finding sites were operating from Brownsville, Texas in the east, to Yuma, Arizona in the west.  The RTU’s operations were integrated with those of a large station in Maine and a small detachment working from the U.S. Embassy in Mexico City – all of which nearly blanketed German communications with and within Central American and Mexico.

For today’s intelligence professionals, especially those participating in the Information Age and supporting  military operations, the saga of the RTU’s illustrates that current goals and practices are part of a tradition traceable to the dusty border sites of 80 years ago.

By Bob Hanyok

Bob Hanyok is a historian with the NSA/CSS Center for Cryptologic History.  He is a graduated of the University of Maryland and served in the Army Security Agency from 1972 to 1975.