from two divisions to seven, making 70,000 additional troops available to augment the 200,000 combat and 100,000 support troops already in South Vietnam. The PAVN high command also recognized the need for improvements to their logistical network to facilitate the transport of sufficient supplies of food, weapons, and ammunition necessary for continuous large-scale operations. By 1973 the 559th Transportation Group, which controlled the Ho Chi Minh Trail in southeastern Laos, was ordered to expand east of the Trường Sơn Mountains and directly into South Vietnam. The new logistical route, Corridor 613, ran inside South Vietnam from the DMZ to all the way to Lộc Ninh. Besides creating the new extension, the 559th upgraded its entire network, constructing all-weather, hard-surfaced roads to accommodate the modern mechanized army that had been rebuilt since the Paris Accords. The work required two years to complete, but the time required for the transport of personnel from North Vietnam to the southernmost seat of battle was reduced from four months to three weeks. One of the most threatening features of the new North Vietnamese build-up was the air defense network that was established within South Vietnam, which by 1975 consisted of twenty-two regiments equipped with radar-controlled gun systems and formidable SA-2 Guideline and shoulder-launched SA-7 Grail anti-aircraft missiles. Such systems posed a major deterrent to the RVNAF, since its aircraft were not equipped to deal with such threats. As a result, RVNAF aerial interdiction of the communist logistical build-up became almost impossible and reconnaissance flights were held to a minimum. This lack of active intelligence collection made estimation of North Vietnamese strength and intentions much more difficult. However, deficiencies of the PAVN's armored and heavy artillery forces, essential for attacks on heavily fortified ARVN regimental and divisional base camps, weighed heavily on the minds of the General Staff planners who drew up the plan presented to the Politburo in October 1974. Much attention was focused on the role ARVN's ammunition shortages played in the collapse of South Vietnam, but it is not known if the PAVN suffered similar shortages. Soviet and Chinese military aid, especially in the category of "offensive weapon" (armor and artillery), declined significantly since the Paris Accords. Also, much of PAVN's armor and artillery was in poor condition, and spare parts were in short supply. Most PAVN artillery units, especially in the South, were still equipped only with light mortar, recoilless gun, or