Vietnamese to reinforce their anti-aircraft defenses of the trail system and aircraft losses rose proportionately. By 1969, the North Vietnamese had also worked out their doctrine and techniques for dealing with the recon teams. Originally, the PAVN had been caught unprepared and had been forced to respond in whatever haphazard manner local commanders could organize. Soon, however, an early warning system was created by placing radio-equipped air watch units within the flight paths between the launch sites and Base Areas. Within the Base Areas, lookouts were placed in trees and platforms to watch likely landing zones while the roads and trails were routinely swept by security forces. The PAVN also began to organize and develop specialized units that would both drive and then fix the teams so that they could be destroyed. By 1970, they had created a layered and effective system, and SOG recon teams found their time on the ground both shortened and more dangerous. The mauling or wiping out of entire teams began to become a more common occurrence. Laos and Cambodia Since his election in 1968, President Richard M. Nixon had been seeking a negotiated settlement to the Vietnam War. In 1970, he saw an opportunity to buy time for the Saigon government during Vietnamization, the phased withdrawal of U.S. troops that began in the previous year. He also sought to convince Hanoi that he meant business. That opportunity was provided by the overthrow of Cambodia's Prince Sihanouk by the pro-American General Lon Nol. Nixon had escalated U.S. involvement in Cambodia by authorizing the secret Operation Menu bombings and by the time of Sihanouk's ouster, the program had been in operation for 14 months. Lon Nol promptly ordered North Vietnamese personnel out of the country. North Vietnam responded with an invasion of the country launched at the explicit request of the Khmer Rouge following negotiations with Nuon Chea. Nixon then authorized a series of incursions by U.S. and South Vietnamese ground forces that began on 30 April. With intelligence on communist Base Areas in eastern Cambodia gleaned from MACV-SOG, huge stockpiles of PAVN arms, ammunition, and supplies were overrun and captured. In May, Operation Freedom Deal, a continuous aerial campaign against the PAVN/Viet Cong and the Khmer Rouge was initiated. SOG recon teams in Cambodia now had all the air support that they needed. As a result of U.S. political reaction, on 29 December the Cooper-Church