Week of August 18

This Week In History

In mid-August 1966, it had become clear to U.S. military leaders that the ROLLING THUNDER bombing campaign required better guidance in order to accurately find and hit targets in far northern North Vietnam. The Central Intelligence Agency helped solve this technological challenge by establishing a secret tactical air control and navigation station in Laos, just miles from the North Vietnamese border. CIA and Air Force personnel built the tiny station, known as Lima Site 85, on top of Phou Pha Thi Mountain. Phou Pha Thi was remote and isolated, with nearly vertical cliffs on three sides, which made it extremely difficult to reach except by helicopter.

The men who manned Lima Site 85 were CIA personnel and Air Force Airmen, who posed as civilian employees of Lockheed Aircraft in order to maintain plausible deniability for the United States of the station’s purpose. Their mission was codenamed Project Heavy Green. They used what at the time was advanced radar navigation technology known as Combat Skyspot to guide U.S. fighters and bombers to their targets in the Đồng bằng sông Hồng (Red River delta) and Hanoi areas of North Vietnam. The entire operation remained a highly classified secret because Laos, where Lima Site 85 was located, was officially neutral in the war.

The tactical air control and navigation station on Phou Pha Thi Mountain remained in continuous operation into 1968, accessible only by helicopter. North Vietnamese intelligence gradually became aware of Lima Site 85’s existence and purpose, though it’s remote location made it extremely difficult to attack from the ground. In January 1968, as part of the operations leading up to the Tet Offensive, Pathet Lao (a Laotian Communist group) and North Vietnamese forces converged on Phou Pha Thi Mountain, aiming to finally destroy the station. The CIA and Air Force directed numerous airstrikes against the attackers, and the station’s defenders managed to hold them off until March 1968, when a small group of North Vietnamese special forces scaled the mountain under the cover of night and overran the station and its Hmong and American defenders. Some of the station’s personnel were rescued by helicopter, but 11 Americans died in the fighting, and Lima Site 85 was permanently lost.¹

¹Timothy Castle, One Day Too Long (New York: Columbia University Press, 1999), 38, 43-45, 59-63, 76–78; Carl Berger, ed. The United States Air Force in Southeast Asia: An Illustrated Account (Washington, D.C.: Office of Air Force History, 1977), 126; Spencer C. Tucker, ed. The Encyclopedia of the Vietnam War: A Political, Social, and Military History, (2nd edition; Santa Barbara, CA: ABC-CLIO, 2011), 657–658; “The Fall of Lima Site 85: The War in Laos,” https://www.cia.gov/library/center-for-the-study-of-intelligence/csi-publications/csi-studies/studies/95unclass/Linder.html (accessed 7/22/14). James C. Linder, “The War in Laos -The Fall of Lima Site 85,” 81–83, https://www.cia.gov/library/center-for-the-study-of-intelligence/kent-csi/vol38no5/pdf/v38i5a09p.pdf (accessed 4/10/2014); Wayne Thompson, To Hanoi and Back: The United States Air Force and North Vietnam, 1966–1973 (Washington, D.C.: Air Force History and Museums Program, 2000), 102–3; Jacob Van Staaveren, The United States Air Force in Southeast Asia: Interdiction in Southern Laos, 1960–1968 (Washington, D.C.: Center for Air Force History, 1993), 289.