When the North Korean People’s Army (NKPA) invaded South Korea in June 1950, the U.S. reaction was swift– air, ground, and naval forces were deployed to help repel the invaders.

Although the North Koreans conquered all of South Korea except a pocket centered on the city of Pusan, an amphibious landing at the port of Inch’on – sometimes called the greatest feat of American arms in this century–coupled with a breakout from the “Pusan Perimeter,” smashed the North Korean army. Under General Douglas MacArthur, American and South Korean forces began advancing northward toward the Yalu River, where the Korean peninsula borders China.

After the Inch’on landing, SIGINT reflected the efforts of fleeing North Korean units to regroup. The collapse of the NKPA was illustrated vividly in SIGINT, as its communications dwindled to a single active network. Many North Korean messages described defense lines, including official encouragement to “fight to the death!”


The Truman administration believed that the war in Korea had been provoked by the Soviet Union, and it feared that the invasion might be the opening move in World War III. Many believed the war in Korea was a strategic move to draw American strength away from Europe, where the Soviets had further aggressive plans. Although these beliefs kept the administration cautious, the likelihood of full victory in Korea kept the Defense Department and Joint Chiefs of Staff from stifling General MacArthur’s desire to push his forces to the Chinese border.

In October, advance South Korean and American army units operating in northern Korea encountered Chinese Communist forces. These encounters were ambiguous and led American commanders to conclude that Chinese troops had not entered Korea in strength. Their estimates were wrong: in November, massive Chinese forces pounced on U.S. and South Korean units with devastating effect. Loss of life was great, and many units began a massive “bug-out” southward. It became one of the worst defeats ever inflicted on the U.S. military.

The sudden and disastrous intervention of Chinese Communist forces in Korea was seen by many in the American government and media as a clear intelligence failure (compounding an original intelligence failure to predict the outbreak of war in June). In actuality, it was a failure to utilize already available intelligence.

In May and June 1950, the Armed Forces Security Agency (AFSA)–NSA’s predecessor–reported the movement of 70,000 troops of China’s People’s Liberation Army (PLA) from two army groups moving down the Yangtze River. On 17 July AFSA reported that China’s best combat force, the 4th Field Army, might be preparing either for operations against Taiwan or for movement to Manchuria. Some elements were then observed moving to Manchuria.

In September, AFSA reported that the PRC had moved two armies to Manchuria and had upgraded local defense forces to main force status. SIGINT also revealed the ongoing movement of artillery units northward by railroad. This movement continued in October. The PLA 39th Anny, usually seen in Wuhan in Central China, was addressed at a location just eighty miles northwest of the Korean border.

AFSA also published additional SIGINT reflections of transfers of Chinese aircraft to Manchuria, with the establishment of new air bases in north China. The Soviet Union, which shared a small stretch of border with both China and Korea, had increased air patrols.

General MacArthur

At the national level, officials at the Departments of Defense and State were concerned about these and collateral reports about Chinese military moves, but they decided that China’s actions were primarily defensive. It was believed that if China intervened in Korea, it would be merely to protect border sites along the Yalu River or perhaps to hold a small enclave in which the North Korean communist government could continue to operate. It is unclear what intelligence officers in Tokyo, where General MacArthur had his headquarters, thought about the AFSA reports. General Willoughby, MacArthur’s longtime G-2, was known to have a poor opinion of SIGINT, and MacArthur’s own record in using SIGINT in World War II was spotty.


A SIGINT report in November that China had ordered 30,000 maps of Korea for its forces in Manchuria failed to change many, if any, minds about PRC intentions in Korea. It should be noted that 30,000 maps was the number required for thirty PLA divisions–as it turned out, just about the troop strength with which China intervened in Korea.

Militarily, the entrance of the Chinese People’s Volunteers (as the PRC government called them) created a new war. This was a new war for SIGINT units as well.  However, traffic analysis made it possible to identify all Chinese armies in Korea and most artillery divisions. Eventually, analysts were able to reconstruct most of the PLA order of battle from Chinese and North Korean messages.

Much of the information about the movement of troops from central China to the regions of Manchuria near the Korean border came from innocuous messages, many of them sent in plain text. These communications, sometimes referred to as “Hi, Mom!” messages because they were personal or administrative, yielded a wealth of data about unit names, locations, and movements.

Whatever the failure to produce or use signals intelligence at the strategic level on the Chinese intervention in Korea, once the PLA began operations south of the Yalu, U.S. SIGINT began a very fruitful period producing tactical intelligence on localized operations.

Source: NSA

Note: The featured picture is the Korean War Memorial in Washington, DC.