The Battle of Lima Site 85 was a battle of the Vietnam War. The site was located at Phou Pha Thi—a mountain 15 miles (24 km) from the border of the Democratic Republic of Vietnam (DRV or North Vietnam) and 30 miles (48 km) from Sam Neua, capital of the Pathet Lao. The term "Lima Site" was derived from the American acronym for map designations of "Landing Sites" within the Secret War zone of the Second Indochina War, an active covert battleground in the larger Cold War.
The remote mountain in northeastern Laos known as Phou Phathi to the Laotians and "the Rock" to the Americans who served there is a dramatic 5,600-foot ridgeline that is just flat enough on top to support a few buildings built into the rock of the mountain and a small clearing that was used as a helicopter landing site. Located 100 miles south of Dien Bien Phu, 160 miles west of Hanoi, and just 25 miles from the PL capital of Samneua, Phou Phathi was a place of great religious significance to the local Hmong and Yao tribesmen. They believed the forbidding mountaintop was inhabited by great phi, or spirits, who exercised supernatural control over the lives and circumstances of the Hmong people. US Ambassador to Laos William Sullivan, however, believed the mountain was a poor choice for the location of secret Air Force navigational equipment because it was too close to Samneua and the PL.