<P> The 559th Group was made the equivalent of a Military Region in 1970 and again placed under the command of General Đồng Sỹ Nguyên . The unit was reorganized into five divisional headquarters: the 470th, 471st, 472nd, 473rd, and the 571st . The group consisted of four truck transportation regiments, two petroleum pipeline regiments, three anti-aircraft artillery (AAA) regiments, eight engineer regiments, and the 968th Infantry Division . By the end of 1970 the 559th was running 27 "Binh Trams", which transported 40,000 tonnes of supplies with a 3.4% loss rate during the year . </P> <P> These supplies traveled in convoys from North Vietnam in relays, with trucks shuttling from only one way - station to the next . The vehicles were then unloaded and reloaded onto "fresh" trucks at each station . If a truck was disabled or destroyed, it was replaced from the assets of the next northern station and so on until it was replaced by a new one in North Vietnam . Eventually, the last commo - liaison station in Laos or Cambodia was reached and the vehicles were unloaded . The supplies were then cached, loaded onto watercraft, or man - portered into South Vietnam . </P> <P> Due to the increased effectiveness of "Commando Hunt", North Vietnamese transportation units usually took to the roads only at dusk with the peak in traffic coming in the early hours of the morning . As American aircraft came on station, traffic would subside until just before dawn, when fixed - wing gunships and night bombers returned to their bases . The trucks then began rolling again, reaching another peak in traffic around 06: 00 as drivers hurried to get into truck parks before sunrise and the arrival of the morning waves of U.S. fighter bombers . By the last phase of "Commando Hunt" (October 1970--April 1972), the average daily number of U.S. aircraft flying interdiction missions included 182 attack fighters, 13 fixed - wing gunships, and 21 B - 52s . </P> <P> The North Vietnamese also responded to the American aerial threat by the increased use of heavy concentrations of anti-aircraft artillery . By 1968 this was mainly composed of 37 mm and 57 mm radar - controlled weapons . The next year, 85 mm and 100 mm guns appeared, and by the end of Commando Hunt, over 1,500 guns defended the system . </P>

When was the ho chi minh trail created