Amendment was passed by Congress, prohibiting participation by U.S. ground forces in any future operations in either Cambodia or Laos. U.S. participation in Cambodian operations (which were already being turned over to all-Vietnamese teams) ended on 1 July 1970 and the same stipulation was to apply in Laos no later than 8 February 1971 (the only qualifications to the restrictions, in both operational areas, were in case of either POW rescue missions or aircraft crash site inspections). Although unknown to the U.S. public, many MACV-SOG veterans participated in Operation Ivory Coast, the Son Tay POW camp raid carried out in North Vietnam on 21 November 1970. The deputy commander of the joint rescue force was Colonel Arthur "Bull" Simons, who had created SOG's cross-border effort in 1965. By 1971 the U.S. was steadily withdrawing from Southeast Asia. As a test of Vietnamization, Washington decided to allow the South Vietnamese to launch Operation Lam Son 719, the long-sought incursion into Laos whose aim would be the cutting the Ho Chi Minh Trail. MACV and the South Vietnamese had been planning just such an operation as far back as August 1964, but the concept was continuously turned down due to the fallout that would have been incurred by the invasion of supposedly "neutral" Laos. The Laotian government (supported by Ambassador Sullivan and the State Department) was adamantly opposed to such an operation. On 8 February, 16,000 (later 20,000) South Vietnamese troops, backed by U.S. helicopter and air support, rolled into Laos along Route 9 and headed for the PAVN logistical hub at Tchepone. Unlike the Cambodian incursion, however, the North Vietnamese stood and fought, gradually mustering 60,000 troops. By 25 March, the South Vietnamese forces retreated. Ironically, MACV-SOG's role in the operation was only peripheral. Recon teams conducted diversionary operations prior to the invasion and helped cover the South Vietnamese withdrawal, but they were otherwise forbidden from participation in the very operation that both MACV-SOG and MACV had come to consider its raison d'etre. In Laos, the North Vietnamese cleared their logistical corridor to the west for security reasons and increased their aid and support for the Pathet Lao. Fighting that once was seasonal became continuous and conventional. The Cambodian Civil War would escalate with the PRC backed Khmer Rouge (also backed by the exiled Sihanouk), fighting Lon Nol's central government. Following US withdrawal from Indochina, its allies in Laos and Cambodia