The term "Lima Site" came from the fact that covert CIA landing strips in Laos were numbered and preceeded by the letter "L" which in the phonetic alphabet is "Lima". This facility started out as nothing more than a short, rough landing field accessible only to a Swiss-made Short Takeoff and Landing (STOL) aircraft. The landing strip was used to ferry supplies and personnel to aid the local Hmong tribesmen who were the CIA's paid mercenaries in the (not so) secret war in Laos against the NVA and Pathet Lao guerillas. Lima Site 85
At some point it was realized that from the top of Hill 1786, named Phou Phathi, line of site communications could be achived with aircraft flying as far away as the skies above Hanoi. The U.S. Air Force received permission to construct a UHF TACAN (TACtical Aid to Navigation) site atop this peak to provide accurate navigational signals to US aircraft flying to and from North Vietnam. Several U.S. Air Force technicians were "sheep dipped" and sent to the site to maintain the navigational beacon.
Sometime afterwards, the U.S. Air Force decided that it needed radar bombing assistance for its planes which had to conduct missions over North Vietnam in weather that precluded visual identification of the target. The solution was to install a radar bomb scoring system, modified to allow it to assist in combat bombing missions, to be installed at this site, since the elevation and close proximity to the North Vietnamese border would allow it to "see" and track bombers over Hanoi and give them precise position information so they could deliver their bomb loads with greater accuracy.
The Air Force had concluded that because of the incredibly steep sides of the mountain the site was safe from attack by the NVA. Some 1,000 CAS (Controlled American Sources), in the form of Hmong tribesmen, were located at the base of the massif to defend against NVA or Pathet Lao attacks.
It didn't take long for the NVA to realize that the site was causing them to suffer from more accurate US bombing. As a result, on the night of May 10-11, 1968 they sent a 33-man sapper team to scale the steep western face of the mountain. The NVA also launched a simultaneous conventional ground attack against the Hmong militias defending the site. It took less than 6 hours for the NVA to overrun the site, and kill eleven Americans. The NVA later dismantled the radar and shipped it to Moscow.
The U.S. Air Force never tried this again.